We knew. FDR is even on record making statements about how we weren't there to save the Jews. The US had opportunities to bomb the train system being used to transport the Jews and didn't. I can't provide links right now (I'll try to circle back later), but we absolutely knew what was going on. We just didn't want to believe it.

A lot of intel was from ULTRA

It was more complicated than that: the Polish government in exile knew at least as early as 1940 and by at least 1942 those had been corroborated by allies and there were official reports presented to allied governments, the UN, and covered in major newspapers:

https://en.wikipedia.org/wiki/The_Mass_Extermination_of_Jews...

https://timesmachine.nytimes.com/timesmachine/1942/02/13/852...

The catch, of course, is that governments don’t act with a single voice: during war time intelligence agencies would be loathe to confirm anything if it revealed information about their sources, and there were enough anti-semites and Nazi sympathizers that all but the most conclusive evidence might be downplayed or lead to consequences (reportedly, concern losing support in Hungary was a factor in not leading with what was happening to the Jews).

There were rumours of the atrocities, but probably the first public testimonies were the Auschwitz Protocols, based on first-hand experiences of a few concentration camp escapees. (https://en.wikipedia.org/wiki/Auschwitz_Protocols)

Read the document which was the subject of my first link and ask whether rumor was an appropriate word to describe an official report to the UN with eyewitness accounts:

https://archive.org/details/TheMassExterminationOfJewsInGerm...

Thanks, I was not aware of this document. However, its wikipedia page mentions that it was not widely believed at the time. Heck, even Auschwitz Protocols after it did not seem to have much success in reaching the audience (western world) for quite a while.

> It was more complicated than that: the Polish government in exile knew at least as early as 1940 and by at least 1942 those had been corroborated by allies and there were official reports presented to allied governments, the UN, and covered in major newspapers:

I had no idea that the "United Nations" was a thing during WWII (though it appears to be more of a synonym for the allies than the body we're familiar with today).