Meaning?

Hi, Would you please tell me what is the meaning of he was there and the last two are from other sources in the following text:

Six names were listed in one column. Beside each was the year of death, and the far right was an amount of money.The largest sum was $ 2.5 million; the lowest,$500000.Lindsey said, " Non of these figures can be trusted, but ten years ago a retired hit man wrote a long tell- all piece for an on line crime magazine, anonymous of course. He claimed responsibility for the first three murders. His command of the details convinced the FBI he was there. The last two are from other sources.

If we take that he is there means he is right and has committed three murders' then why the last two and not the last three?

Thanks