The former National Security Agency intelligence contractor leaked thousands of classified US documents in 2013 revealing the vast US surveillance put in place after the September 11, 2001 attacks.
Several prominent human rights groups have launched a campaign to convince President Barack Obama to pardon Snowden, who is living in exile in Russia.
The whistleblower is also the subject of an Oliver Stone movie set for release Friday in the United States.
But according to a summary of the two-year report prepared by the House Intelligence Committee, Snowden “was a disgruntled employee who had frequent conflicts with his managers and was reprimanded just two weeks before he began illegally downloading classified documents.”
The report also finds that Snowden “did not voice such concerns to any oversight officials” at the NSA, and he should not be considered a whistleblower protected under law.
Although the 36-page report is classified, officials released a shorter, unclassified version.
Snowden “doctored his performance evaluations and obtained new positions at NSA by exaggerating his resume and stealing the answers to an employment test,” the report said, adding that he took advantage of its access as network administrator to search hard drives on his colleagues’ computers.
“Edward Snowden is no hero — he’s a traitor who willfully betrayed his colleagues and his country,” Intelligence Committee Chairman Devin Nunes said.
Snowden put US service members “and the American people at risk,” he added.
NSA and Cybersecurity Subcommittee Chairman Lynn Westmoreland said Snowden “did more damage to US national security than any other individual in our nation’s history.”
Snowden reacts on Twitter
Reacting on Twitter, Snowden addressed several specific allegations in the report before categorically dismissing the document.
“Bottom line: after ‘two years of investigation,’ the American people deserve better,” he wrote. “This report diminishes the committee.”
“Congress spent two years writing a report to discourage you from going to see this film,” he tweeted later, referring to the Oliver Stone movie. “It opens tomorrow.”
Following Snowden’s revelations, widespread outrage prompted Congress to adopt measures in June 2015 to regulate the NSA’s collection of Americans’ phone call metadata.
However, White House press secretary Josh Earnest dismissed pressure for a pardon on Wednesday, saying Snowden would enjoy legal due process at a trial in the United States, where he faces up to 30 years in prison for espionage and theft of state secrets.
“His conduct put American lives at risk and it risked American national security,” he told reporters. “And that’s why the policy of the Obama administration is that Mr Snowden should return to the United States and face the very serious charges that he’s facing.”