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TLDR: Så tidigt som 1933 men säkert före den 24 november 1942.

Det finns mycket debatt om ämnet för din fråga. Följande är från Holocaust Encylopedia

US STATE DEPARTMENT RESPONSE TO NEWS OF THE “FINAL SOLUTION”

In August 1942, the State Department received a report sent by Gerhart Riegner, the Geneva-based representative of the World Jewish Congress (WJC). The report revealed that the Germans were implementing a policy to physically annihilate the Jews of Europe. Department officials declined to pass on the report to its intended recipient, American Jewish leader Stephen Wise, who was President of the World Jewish Congress.

Despite the State Department's delay in publicizing the mass murder, that same month Wise received the report via British channels. He sought permission from the State Department to make its contents public. Undersecretary of State Sumner Welles asked Wise not to publicize the information until the State Department confirmed it. Wise agreed and after three months the State Department notified him that its sources had confirmation. On November 24, 1942, Wise held a press conference to announce that Nazi Germany was implementing a policy to annihilate the European Jews. A few weeks later, on December 17, the United States, Great Britain, and ten other Allied governments issued a declaration denouncing Nazi Germany's intention to murder the Jews of Europe. The declaration warned Nazi Germany that it would be held responsible for these crimes.

US PRESS COVERAGE OF THE “FINAL SOLUTION”

During the era of the Holocaust, the American press did not always publicize reports of Nazi atrocities in full or with prominent placement. For example, the New York Times, the nation's leading newspaper, generally deemphasized the murder of the Jews in its news coverage. The US press had reported on Nazi violence against Jews in Germany as early as 1933. It covered extensively the Nuremberg Laws of 1935 and the expanded German antisemitic legislation of 1938 and 1939. The nationwide state-sponsored violence of November 9-10, 1938, known as Kristallnacht (Night of Crystal), made front page news in dailies across the US as did Hitler's infamous prediction, expressed to the Reichstag (German parliament) on January 30, 1939, that a new world war would mean the annihilation of the Jewish “race.”

As the magnitude of anti-Jewish violence increased in 1939-1941, many American newspapers ran descriptions of German shooting operations, first in Poland and later after the invasion of the Soviet Union. The ethnic identity of the victims was not always made clear. Some reports described German mass murder operations with the word "extermination." As early as July 2, 1942, the New York Times reported on the operations of the killing center in Chelmno, based on sources from the Polish underground. The article, however, appeared on page six of the newspaper. Although the New York Times covered the December 1942 statement of the Allies condemning the mass murder of European Jews on its front page, it placed coverage of the more specific information released by Wise on page ten, significantly minimizing its importance.

TL; DR: Diskriminerande politik mot judar kom långt före dödslägerna, och USA var väl medveten om dessa politik i mitten av 30-talet. Den amerikanska regeringen blev medveten om internering av judar strax efter det att denna politik antogs också. Förekomsten av dödsläger uppmärksammades av de allierade regeringarna senast 1942, men förstod inte fullt ut först. Vid tiden för Normandie-landningarna var många amerikanska medborgare, särskilt amerikanska judar, väl medvetna om rykten om sådana anläggningar.

Innan förintelsen:

USA var medveten om nazisternas förtryck och brutalitet gentemot judarna strax efter att Hitler tog makten.

In 1933, new German laws forced Jews out of their civil service jobs, university and law court positions, and other areas of public life. In April 1933, laws proclaimed at Nuremberg made Jews second-class citizens. These Nuremberg Laws defined Jews, not by their religion or by how they wanted to identify themselves, but by the religious affiliation of their grandparents. Between 1937 and 1939, new anti-Jewish regulations segregated Jews further and made daily life very difficult for them. Jews could not attend public schools; go to theaters, cinema, or vacation resorts; or reside or even walk in certain sections of German cities.
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USA och de andra framtida allierade krafterna gjorde lite för att motsätta sig de tidiga tecknen på anti-judisk politik i nazistiska Tyskland.

In 1933 the Nazi Party under the leadership of Adolf Hitler legally gained power in Germany, and it did not take long for the persecution of Jews to begin. On April 7, 1933, the Reichstag, under Hitler’s influence, adopted the Restoration of the Professional Civil Service Law that dismissed non-Aryans from governmental positions.

This restoration of this act was more significant than it appeared because the Civil Service decree included bankers, lawyers, railroad and hospital workers, medicine, law, schools, universities, and the arts. The Nazi intent of these laws was to eliminate Jewish influence in Germany.

Not long after the Nazis implemented the Civil Service laws in Germany the first significant protest against Jewish mistreatment came from the American Jewish Committee. This was a committee developed to protect the rights of Jews, and they asked the American government to properly investigate what was happening in Germany. The American Jewish Congress began protests and had a strong membership and following that would help inform the rest of the country of the worsening conditions in Germany. For the United States to protest another country’s treatment of its citizens during the era of segregationist Jim Crow laws was somewhat hypocritical. The United States had no basis to criticize another country for racial injustice when it practiced similar discrimination of certain races of people. Nonetheless protests began and Christian groups, such as the American Christians, made public protests against Nazi anti-Semitic acts with the support of influential public figures such as Alfred E. Smith, Newton D. Baker, and John W. Davis. Slowly information about Nazi anti-Semitic acts began to filter into the United States. The State Department after hearing of these allegations asked the American Embassy in Berlin to investigate these accusations. The embassy liaison reported that there was good indication that the Nazis were hiding something. When the Germans were questioned on this issue of Jewish mistreatment, the Nazis claimed this anti-Semitic rhetoric and attacks were isolated incidents and they were in process of being stopped. This was very early in Hitler’s regime and the true magnitude of his hatred of Jews was not yet identified. Ranking United States government individuals probably knew that Hitler inherently disliked Jews because of his biography Mein Kamp, but there was no precedent of Jewish genocide to use as a guide to future events.

Throughout 1933 reports of Jewish mistreatment at the hands of the Nazis kept surfacing. H.R. Knickerbocker, who was the New York Evening Post correspondent in Berlin, reported in April 1933 that an undetermined number of Jews had been killed, or fled, or been deprived of their livelihood in the Reich. In this Newsweek article Mr. Knickerbocker made two interesting observations. The first was that he did not understand why the Nazis had such hatred for the Jews, and secondly he concluded that the Germans were envious of Jewish accomplishments in Germany. These two conclusions would prove to be very astute in the years that followed, yet few people in the United States seemed to understand what was happening to the Jews in Germany. From the everyday citizen to the higher powers in the government this hatred was not fully understood until it was too late. The Nazis under Hitler’s leadership ignited the hidden anti-Jewish attitudes of gentiles in Germany. Hitler was a master orator and he convinced the German citizens that the Jews were the reason that Germany lost World War I and fell into economic despair in the 1920s.
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Det enklaste sättet för andra länder att hjälpa tyskarnas judar var att öka invandringskvoterna, men det var inte politiskt genomförbart i många år, för världen - inklusive USA - var fortsatt förknippad med den stora depressionen och allmänhetens känsla var fokuserat på att hitta jobb för amerikanska medborgare på bekostnad av alla andra problem. Vidare var antisemitism överflödig över Europa och USA vid den tiden, så sympati för judarna om Tyskland var ganska svårt att hitta.

Many German and Austrian Jews tried to go to the United States but could not obtain the visas needed to enter. Even though news of the violent pogroms of November 1938 was widely reported, Americans remained reluctant to welcome Jewish refugees. In the midst of the Great Depression, many Americans believed that refugees would compete with them for jobs and overburden social programs set up to assist the needy.
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Och:

Like most other countries, the United States did not welcome Jewish refugees from Europe. In 1939, 83% of Americans were opposed to the admission of refugees.3 In the midst of the Great Depression, many feared the burden that immigrants could place on the nation’s economy; refugees, who in most cases were prevented from bringing any money or assets with them, were an even greater cause for concern. Indeed, as early as 1930, President Herbert Hoover reinterpreted immigration legislation barring those “likely to become a public charge” to include even those immigrants who were capable of working, reasoning that high unemployment would make it impossible for immigrants to find jobs.

While economic concerns certainly played a role in Americans’ attitudes toward immigration, so too did feelings of fear, mistrust, and even hatred of those who were different. Immigration policies were shaped by fears of communist infiltrators and Nazi spies. Antisemitism also played an important role in public opinion.
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Av dessa skäl många historiker med rätta hävda att andra europeiska nationer , och USA delar del av skulden för förintelsen.

Under förintelsen:

Senast 1940 var rapporter om Auschwitz i synnerhet anlända till London via Polskt motstånd och den polska regeringen i exil.

1942 lämnade den polska regeringen i förflutet ett dokument som beskriver massutrotning av judar i ockuperat Polen. Det var inte det första sådana papperet som de hade producerat, men det var det första som publicerades.

Note to the Governments of the United Nations - December 10th, 1942

Ponizszy dokument informuje wszystkie Narody Zjednoczone o exterminacji Zydow polskich i europejskich przez Hitlerowcow na terenach okupowanych.

INTRODUCTORY NOTE
The purpose of this publication is to make public the contents of the Note of December 10th, 1942, addressed by the Polish Government to the Governments of the United Nations concerning the mass extermination of Jews in the Polish territories occupied by Germany, and also other documents treating on the same subject.
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En vecka senare omfattade New York Times papperet:

ALLIES CONDEMN NAZI WAR ON JEWS; United Nations Issue Joint Declaration of Protest on 'Cold-Blooded Extermination' 11 ALLIES CONDEMN NAZI WAR ON JEWS
December 18, 1942

WASHINGTON, Dec. 17 -- A joint declaration by members of the United Nations was issued today condemning Germany's "bestial policy of cold-blooded extermination" of Jews and declaring that "such events can only strengthen the resolve of all freedom-loving peoples to overthrow the barbarous Hitlerite tyranny."
- NY Times

Här är den fullständiga texten till FN: s gemensamma förklaring:

"The attention of the Belgian, Czechoslovak, Greek, Jugoslav, Luxembourg, Netherlands, Norwegian, Polish, Soviet, United Kingdom and United States Governments and also of the French National Committee has been drawn to numerous reports from Europe that the German authorities, not content with denying to persons of Jewish race in all the territories over which their barbarous rule has been extended, the most elementary human rights, are now carrying into effect Hitler's oft-repeated intention to exterminate the Jewish people in Europe.

From all the occupied countries Jews are being transported in conditions of appalling horror and brutality to Eastern Europe. In Poland, which has been made the principal Nazi slaughterhouse, the ghettos established by the German invader are being systematically emptied of all Jews except a few highly skilled workers required for war industries. None of those taken away are ever heard of again. The able-bodied are slowly worked to death in labor camps. The infirm are left to die of exposure and starvation or are deliberately massacred in mass executions. The number of victims of these bloody cruelties is reckoned in many hundreds of thousands of entirely innocent men, women and children.

The above-mentioned governments and the French National Committee condemn in the strongest possible terms this bestial policy of cold-blooded extermination. They declare that such events can only strengthen the resolve of all freedom-loving peoples to overthrow the barbarous Hitlerite tyranny. They reaffirm their solemn resolution to insure that those responsible for these crimes shall not escape retribution, and to press on with the necessary practical measures to this end."
- Wikipedia

I maj 1943 beslutade en Allied konferens i Bermuda att inte öka invandringskvoter i USA och Storbritannien för europeiska Judar flyr från nazistiska ockupation och fördömer faktiskt många människor i döden i lägerna. Nästa vecka, den amerikanska zionistiska kommittén för en judisk armé publicerade en del i New York Times, fördömde beslutet:

Viddenhärtidenvardeallieradeförstareaktionen-avvisandeavrapporternaommassutrotningsompolskpropaganda-ohållbar.Underdetgångnaårethadedeallieradeunderrättelsetjänstenvarit avlyssning och avkodning av tyska poliskommunikationer som beskriver intag och dödsrekord från många koncentrationsläger . Dessa avlyssnade meddelanden kallas Bletchley Park Concentration Camp decodes .

Katten var ute av väskan, och den vanliga pressen började rapportera om nazistpolicyer för massutrotning oftare efter 1943.

Slutsats:

Det hade varit möjligt för en genomsnittlig medborgare / soldat från USA att förbli mest omedvetna om förintelsen, men nästan alla skulle ha varit medvetna om nazistiska förföljelser av judar före 1944 och många skulle ha hört åtminstone några rykten eller rapporter om massutrotning. Amerikanska judar skulle ha varit mycket mer akut medvetna om dessa berättelser och skulle troligen behandla nazisterna som deras fiender på en djup personlig nivå - mycket mer än gentile amerikaner, som var mer mot Japan än Tyskland, gjorde.

Det var en kombination av att komma igenom helvetet (kan du föreställa dig att du körde upp den stranden med människor som döde åt vänster och höger?!?) och levande, liksom det faktum att han bara hade hjälpt att bryta genom en stor försvarslinje och besegrade människor som var ansvariga för hundratusentals (vid den tidpunkten) av sitt folks dödsfall.

Jag är ingen psykolog (även om jag bodde på Holiday Inn en gång), men jag tror att när adrenalinen att komma genom den stranden hade det, kommer all rädsla och ilska att träffa dig och det kommer att orsaka vissa känslor. Om du inte är känslomässig efter det är du numb.