Adrián Lamo, a hacker known for transmitting information that led to the arrest of Chelsea Manning, died at the age of 37.
In online messaging conversations, Manning confided in him, describing confidential military material that Manning had sent to Wikileaks.
Wikileaks released the video of an American helicopter that killed seven people, including a journalist who works for the Reuters news agency.
The cause of Lamo's death, confirmed to the BBC by the Sedgwick County coroner in Kansas. It has not yet been made public.
On Facebook, his father Mario wrote: "With great sadness and a broken heart I have to let all Adrian's friends know and absolve [sic] that he is dead." A brilliant mind and compbadionate soul has gone, he was my beloved son. "
Lamo's own record as a hacker included some high-profile targets, such as Microsoft and the New York Times.
& # 39; Push on me & # 39;  Speaking to The Guardian newspaper in 2011, Lamo described his decision to renounce Manning as "not one that I decided to make, but pushed me".
Lamo said he would have a "lasting remorse" if Manning gave him a long sentence.
Manning, known at the time as Bradley Manning, was finally sentenced to 35 years in prison. However, President Barack Obama later commuted her sentence and was released in May 2017.
Now she is trying to become the senator from Maryland, her home state.
Wikileaks founder Julian Assange described Lamo on Friday as a "petty" swindler and traitor of basic human decency. "