Popov had earlier supplied information after the German disaster at Tarino on Sicily. In that attack, the British used carrier based aircraft to annihilate a good portion of the German fleet. The Japanese quickly sent over a team to assess the damage done, and, more importantly, try and get a better understanding of how the attack was mounted.
A month later Popov had information concerning the Japanese. They were requesting information from their local Hawaiin agents as to the strength and positioning of ships berthed at Pearl Harbor, the progress of the channel dregding, the depth of all channels and anti aircraft capabilities.
This information, coupled with the earlier intelligence, were pretty good indications that Japan was going to attack Pearl Harbor. It was only a question of when.
This information became the property of J. Edgar Hoover at the FBI. He was not pleased with the Presidents directive that he share information with the British. He was a notorious Anglophobe, which is to say that he, like the Japanese Colonel Sato in "A Bridge Over the River Kwai", hated the British. So, this info was locked away in August of 1941, having been dismissed by the Director as "nonesense."
In reality he was disgusted with Popovs playboy lifestyle and had even told others on more than one occassion that he considered the Yugoslavian to "be a double agent, using German and American money to live like a degenerate playboy."
And thus was born that old story about how FDR knew in advance of Pearl Harbor and did nothing. J. Edgar Hoover told him it didn't matter. That was in August.
