Federal authorities are investigating whether sensitive data was stolen from congressional offices by several Pakistani-American tech staffers and sold to Pakistani or Russian intelligence, knowledgeable sources say.

What started out 16 months ago as a scandal involving the alleged theft of computer equipment from Congress has turned into a national-security investigation involving FBI surveillance of the suspects.

Investigators now suspect that sensitive US government data — possibly including classified information — could have been compromised and may have been sold to hostile foreign governments that could use it to blackmail members of Congress or even put their lives at risk.

“This is a massive, massive scandal,” a senior US official familiar with the widening probe told The Post.

Alarm bells went off in April 2016 when computer security officials in the House reported “irregularities” in computer equipment purchasing. An internal investigation revealed the theft of hundreds of thousands of dollars in government property, and evidence pointed to five IT staffers and the Democratic Congress members’ offices that employed them.

The evidence was turned over to the House inspector general, who found so much “smoke” that she recommended a criminal probe, sources say. The case was turned over to Capitol Police in October.

When the suspected IT workers couldn’t produce the missing invoiced equipment, sources say, they were removed from working on the computer network in early February.

During the probe, investigators found valuable government data that is believed to have been taken from the network and placed on offsite servers, setting off more alarms. Some 80 offices were potentially compromised.

Most lawmakers fired the alleged “ringleader” — longtime IT staffer Imran Awan — in February. But Florida Rep. Debbie Wasserman Schultz, the former Democratic National Committee chief, kept Awan on her payroll until his arrest last month on seemingly unrelated charges of defrauding the congressional credit union.

For more than a decade, Awan, his wife, two relatives and a friend worked for 30 House Democrats. They included New York City pols Gregory Meeks, Joseph Crowley and Yvette Clarke and members of the sensitive Intelligence and Foreign Relations committees.

The Democrats who hired the five suspects apparently did a poor job vetting them. Awan’s brother Abid had a rap sheet with multiple offenses, including a conviction for DWI a month before he was hired, and filed for bankruptcy in 2012.