Polish Captain Witold Pilecki was the man responsible for originally gaining and disseminating information about the death camps, specifically Auschwitz.

Here was a man who actually volunteered to be captured and thrown into Auschwitz (then escaped!). At the time, Auschwitz was believed to have been merely an internment camp.

Pilecki's objectives were to gather intelligence, cause as much disruption as possible, then escape.

Once inside, he discovered the horrifying true nature of the camp. Among many things he did as a prisoner, he set up an intelligence network that, beginning in March of 1941, passed along information about the atrocities to Warsaw, which then passed them along to London.

The reports were the very first record about the holocaust and were the primary source of intelligence for the allies. Witold's Reports would be one of three reports (the other two coming in 1944) known as the Auschwitz Protocols that warned of the mass murders.

They included details about the gas chambers, sterilization experiments and the crematoriums that could burn 10,000 people daily. He mentioned 30,000 people had died in one day and over the three years of reporting, 2 million people total.

The British Office of Strategic Service, for their part, read the report and filed it away, believing it to be grossly exaggerated.

Source(s)/more info:

Meet The Man Who Sneaked Into Auschwitz

English version of Witold's Report

The Auschwitz Volunteer: Beyond Bravery

Hilberg, Raul. The Destruction of the European Jews. Yale University Press, 2003, p. 1212