How did the news of the reality Nazi concentration camps reach the United State's mainstream? What were the waves of awareness and when?

On November 23rd, 1938, the Arizona Republic was one of several papers to carry this stark headline:

Source: Newspapers.com

On November 6th, 1939, on page 7 of the New York Times, under a headline 'Jews said to face famine in Poland' it was reported that

About 1,500,000 Jews remaining in that area are condemned to starvation.

Just over a year later, on November 26th, 1940, the same newspaper had article (on page 8) on the Polish Ghetto under the heading

Walls Will Enclose Warsaw Jews Today; 500,000 Begin 'New Life' in Nazi-Built Ghetto

On August 24th, 1941, Winston Churchill made a radio broadcast which included

Whole districts are being exterminated. Scores of thousands – literally scores of thousands – of executions in cold blood are being perpetrated by the German police-troops upon the Russian patriots who defend their native soil....we are in the presence of a crime without a name.

By this time, the first mass murder of Jews (June 27th) had already begun. How much coverage this speech was given in the US, though, is unclear.

On June 29th, 1942, a New York CBS radio broadcast included the following:

It is now estimated that the Germans have massacred more than one million Jews in Europe since the war began. That's about 1/6 of the Jewish population in the Old World.

Around the same time, the Los Angeles Times wrote:

Nazis kill million Jews, says survey

On December 13th, 1942, Ed Murrow on CBS announced:

What is happening is this: millions of human beings, most of them Jews, are being gathered up with ruthless efficiency and murdered.... The phrase "concentration camp" is obsolete, as out of date as "economic sanctions" or "non-recognition." It is now possible to speak only of "extermination camps."

On December 17th, 1942, a joint declaration by the Allies published on the front page of many newspapers referred to (among other things) Poland being "made the principal Nazi slaughterhouse" and to "mass executions".

Even before these very public announcements, there had been numerous stories in the press about Nazi atrocities.

Yet, public opinion polls showed

that while half of U.S. respondents in 1943 thought the fact that 2 million Jewish Europeans had been murdered was just a rumor, by 1944 about three-quarters believed concentration camps were really part of the Nazi plan — and yet they still couldn’t fathom the number of victims involved.

Unfortunately, many of these stories were 'buried' on the inside pages or otherwise given little prominence, as in the Honolulu Star-Bulletin On December 17, 1942 (see below).

Source: United States Holocaust Memorial Museum

The headline Nazis plan to kill all Jews made it onto the front page of the Endicott Daily Bulletin (below) on November 25, 1942...but one has to look hard to find it.

Source: History Unfolded: US Newspapers and the Holocaust

This limited understanding or acceptance of the then known extent (as reported) of the holocaust was revealed when respondents were asked (in November 1944) how many they thought had been killed in the concentration camps:

33% refused to venture a guess, 36% thought the number killed was under 100,000, 8% between 100,000 and 1,000,000, and 24% thought a million or more.

When photos of the concentration camps liberated by American and British troops in April and May 1945 began to appear, the percent of those polled who believed that millions had died rose to 84%. This was when photographs of

skeletal survivors and stacked corpses, and the wrenching accounts of mass murder became a staple of American publications and newsreels.

Even so, 16% still did not believe. Statistics and opinion polls are all well and good but, as Peter Novick in The Holocaust in American Life says,

How many Americans had knowledge of the Holocaust while it was going on is as much a semantic as a quantitative question. It calls for distinctions among varieties of awareness, consciousness, belief, attention.

Or, as this Time Magazine article puts it:

Looking at the news that publications like TIME ran in the 1930s and ’40s shows that, in fact, Americans had lots of access to news about what was happening to Europe’s Jewish population and others targeted by the Nazi regime. But it also highlights a central truth about this period — and human beings in general. Reading or hearing something is not the same as understanding what it truly means, curator Daniel Greene tells TIME, and there’s a wide “gap between information and understanding.”

Then there is the problem in war of what to believe. It can be hard for the average citizen to distinguish between truthful reporting and propaganda, and many older members of the newspaper profession had memories of being fooled by propaganda during WWI. Nor did people in the 1940s have the internet at their fingertips and instant cable news with often graphic images.

Even William Casey, head of the Secret Intelligence Branch in Europe for the OSS, said:

The most devastating experience of the war for most of us was the first visit to a concentration camp.... We knew in a general way that Jews were being persecuted, that they were being rounded up ... and that brutality and murder took place at these camps. But few if any comprehended the appalling magnitude of it. It wasn't sufficiently real to stand out from the general brutality and slaughter which is war.