Smith told the Journal that previous year he put together a team to track down the more than 30,000 emails that may have been stolen from Clinton's private server when she was Secretary of State.
A medical examiner's report listed the cause of death as "asphyxiation due to displacement of oxygen in confined space with helium".
Peter W Smith, 81, spoke to the Wall Street Journal in May previous year, and he died about 10 days later on May 14.
Smith's body was found with carefully prepared documents, including a suicide note, in which he said he was ill and had a $5 million life-insurance policy about to expire.
Those investigators have examined reports from intelligence agencies that describe Russian hackers discussing how to obtain emails from Mrs. Clinton's server and then transmit them to Mr. Flynn via an intermediary, according to US officials with knowledge of the intelligence. He was 81 at the time of his death.
The cause of Smith's death had previously been a mystery, though it was widely assumed he had died of natural causes. Smith's former associates said he was suffering from health issues and frequently complained of heart problems.
A private-equity executive and a longtime opposition researcher for conservative groups, Smith told Harris last month that he had found several hacker groups-two of them Russian-that claimed to have access to Clinton's e-mails, although he couldn't confirm their authenticity.
Authorities also found a receipt from a local Walmart for a helium tank and pliers.
Smith's account is consistent with the findings from United States investigators examining Russia's involvement in the 2016 U.S. presidential election, according to sources cited by The Journal.
Days before his death Smith, gave an interview to The Wall Street Journal about his attempts to obtain the emails and his relationship to former Trump national security adviser Michael Flynn. Details on whether Flynn was involved in Smith's mission are not known.
The Journal said Flynn did not respond to requests, the White House declined comment, and the campaign said Smith never worked for it and that any such action undertaken by Flynn, if true, was not on its behalf.