Well I had a long answer typed up to this, and then in a copy/paste error, I lost it all. Such is life. I will leave you, though, with this article from the Nov. 24, 1942 issue of the New York Times (page 10, to be exact). It's behind a paywall, so I'll quote it in its entirety:

WISE GETS CONFIRMATIONS; Checks With State Department on Nazis' 'Extermination Campaign' WASHINGTON, Nov. 24 (AP) - Dr. Stephen S. Wise, chairman of the World Jewish Congress, said tonight that he had learned through sources confirmed by the State Department that about half the estimated 4,000,000 Jews in Nazi-occupied Europe had been slain in an "extermination campaign".
Dr. Wise, who also is president of the American Jewish Congress and chairman of a committee composed of representatives of leading Jewish organizations in America, said that the Germans were "even exhuming the dead for the value of the corpses." Dr. Wise spoke at a press conference soon after he had conferred with State Department officials.
He stressed the fact that most of his information came from various sources other than the State Department. He said that his committee had deliberately awaited I confirmation before making public any report on its investigations since it was organized on Labor Day.
"The State Department finally made available today the documents which have confirmed the stories and rumors or Jewish extermination in all Hitler-ruled Europe," Dr. Wise said.
Dr. Wise, accompanied by his son, James Waterman Wise, planned to leave Washington tonight for New York, where he will meet with the Jewish Committee tomorrow afternoon. after which a report of the European conditions will be issued.

This is, far as I know, the first instance of a public declaration in the American media that what the Germans were doing was a coordinated campaign of extermination. It should be noted that the major outlines of the Holocaust had all been reported on in the media prior to this date, including the boycott of Jewish businesses in 1933, the opening of the concentration camp at Dachau n 1933 (the first camp), the Nuremberg Laws in 1935, the Reichspogromnacht of Nov. 1938, the Reichstag Speech of Jan. 30, 1939, the mass shootings of Jews in Poland in 1939-1940, the mass shootings of Jews in the Soviet Union in 1941-42, and the operations of the death camp at Chelmno in 1942.

Source: USHMM