In a report endorsed by both its Republican and Democratic leaders, the House intelligence committee said Snowden was "not a whistleblower" as he has claimed.
Most of the material he stole from the NSA was not about invasions of privacy, but revealed intelligence and defense programs of great interest to America's foreign adversaries, it said.
The committee said that while the "full scope" of damage caused by Snowden's disclosures remains unknown, a review of materials he allegedly compromised "makes clear that he handed over secrets that protect American troops overseas and secrets that provide vital defenses against terrorists and nation-states."
The committee released only a four-page summary of what it said was a 36-page investigative report that remains Top Secret, but the summary contained strong words about Snowden's actions and background.
The report contains previously unreported allegations about Snowden and his possible motives for taking government secrets. It alleges that Snowden, who took refuge in Moscow after fleeing to Hong Kong, "was and remains a serial exaggerator and fabricator."
It says his official employment records show that while he claimed to have left US Army basic training because of broken legs, in fact he "washed out because of shin splits." The report says Snowden claimed to have obtained a high-school graduation equivalent, but in fact never did.
It also says he claimed to have worked as a "senior advisor" for the CIA when in fact he was an entry-level computer technician.
While the report says Snowden "stole 1.5 million sensitive documents," other sources who have examined materials he turned over to media outlets say that the total is between 200,000 and 300,000 documents.
Some US officials involved in Snowden-related investigations acknowledge that the US government does not know how many documents Snowden downloaded, and that the 1.5 million figure is an estimate.
The report also disputes Snowden's motives for taking and leaking classified information, saying he got into a "workplace spat" with NSA managers in June 2012 over how to manage computer updates. It says a contracting officer reprimanded him for failing to follow proper grievance procedures, and he began downloading classified information two weeks later.
However, in 2013, Reuters quoted US officials and other sources familiar with the matter as saying that Snowden began downloading such data in April 2012, almost a year earlier than had previously been reported at the time.
The issuance of the House committee report coincided with the release of "Snowden", a movie directed by Oliver Stone that portrays him as a whistleblower and hero. On Wednesday, prominent human rights advocates urged President Barack Obama to pardon Snowden before leaving office in January.
US officials have said Obama is not considering a pardon for Snowden, who is facing US criminal charges for providing classified information to unauthorised persons.
Ben Wizner, an attorney with the American Civil Liberties Union who represents Snowden, dismissed the House committee report as lacking substance.
Wizner said the report's release a day before the Snowden film opens "is evidence that people in the intelligence community are taking us seriously, that they are concerned that Oliver Stone's movie will help solidify Snowden's image as a true patriot, which he is."