When Ileana Privetera started working for the contractor USIS, the firm that vetted National Security Agency leaker Edward Snowden and Navy Yard shooter Aaron Alexis, it sounded like the perfect job. A mother, she would have flexible hours for her family, and she would be helping the country by running background checks on people who were doing the government’s most critical jobs.

She quickly learned that she was being asked to do the impossible.

A computer-software system spat out assignments for her every week with new people to interview. The deadlines were merciless, with as many as 10 cases due in a single day.

Privetera was driving madly from one end of Los Angeles to another to complete interviews. And she was never sure whether she was asking enough questions as she probed people’s personal lives, asking about their rocky marriages and finances.

“I didn’t like the feeling, ‘Am I doing this right?’ ” said Privetera, who left her job at USIS in December after about five months. “I felt like we were doing something important.”

How OPM processes security clearance applications

The work was so overwhelming that she and co-workers joked about taking anti-anxiety medications.

No evidence has emerged that Falls Church-based USIS cut corners when it vetted Snowden and Alexis. But the company, which has grown to become the biggest private contractor handling background checks for the government, has drawn the notice of lawmakers and the Justice Department.

It is under criminal investigation over whether it misled officials about the thoroughness of its work. A number of former USIS employees have been charged with falsifying records in recent years. And Monday’s Navy Yard shooting is raising questions about how the government vets employees who are given access to some of the country’s most sensitive documents and facilities.

USIS declined to comment for this story, said company spokesman Ray Howell. But in interviews, some former employees describe an environment where people went weeks without seeing their bosses and handled sensitive documents with no supervision in their home offices and occasionally at Starbucks. The goal at all times: volume.

“It was like wink, wink, do this as fast as humanly possible,” said a former USIS investigator, who spoke on the condition of anonymity to avoid crossing a former employer. “There was this intense pressure to do more and faster.”

With about 7,000 employees, USIS handles about 45 percent of all contracted background checks for the Office of Personnel Management. Last year, OPM processed nearly 2.3 million investigations.

The story of how USIS became the biggest private name in background checks is unusual. The company was originally part of OPM. But during the Clinton administration, with the Cold War long over, there was less demand for security clearances. As part of Clinton’s “reinvent government” initiative, the employees of OPM’s security and investigations unit were transferred in 1996 to a private firm, wholly owned by the workers. It was a first for the government.

The move was so revolutionary that many OPM employees and members of Congress vehemently opposed the plan.

“National defense, security, and the fitness and suitability of the Federal workforce are not commodities like hammers, ashtrays, and space toilets to be traded on the open market and sold to the lowest bidder,” said Deborah Abraham Apperson, an OPM employee, in testimony on Capitol Hill in 1996. “Who among us is willing to take the risk of letting a Timothy McVeigh [the Oklahoma City bomber] ‘slip through the cracks’ in order to save a few dollars by cutting corners?”

USIS received an immediate leg up: a noncompetitive three-year contract, according to a 2011 report by the Congressional Research Service. The company quickly dominated the field of private background checks for the government. During the Iraq war, it expanded into training Iraqi police units.

USIS’s lucrative work conducting investigations for the government — worth $334 million this year — is at risk given the criminal probe. Some lawmakers are calling on OPM to cut off the company, which is now owned by Providence, a private-equity firm.

OPM has been under pressure to process background checks more quickly since new standards were put in place by a 2004 intelligence reform law.

As the agency has leaned heavily on contract workers, “the quality of the investigations has gone down,” said Mark Riley, a former Army officer who works as a private security clearance lawyer. “They are much more cursory. They don’t ask the right follow-up questions. . . . The bottom line is the buck, rather than national security.”

Former employees say the relentless demand to churn out background checks meant that even when USIS investigators wanted to do their best to follow up on red flags, there was limited time.

“If I had three months to check this person out . . . I’d be doing a more-thorough process,” said a third former employee, who spoke on the condition of anonymity because she also did not want to upset her former employer. “When you’re giving me a week to interview 50 people, that’s impossible.”

This employee said she was particularly concerned about the process for giving secret-level clearance — the kind issued to Alexis. That level requires only information from a self-reported questionnaire, a credit check and data from local law enforcement. No interviews with the subject or with references, including neighbors and former spouses, are required.

But even for top-secret clearance, where such interviews are required, some former employees said they felt rushed. (Snowden, who leaked top-level NSA documents to The Washington Post and Britain’s Guardian newspaper, had top-secret clearance.)

“It’s very: ‘Here’s a sheet of questions, ask the questions, hurry and get the answers, submit them and move forward,’ ” said one of the former employees. “There’s just not a lot of paying attention to potential red flags and that sort of thing.”

Candidates do not have to disclose mental-health counseling they have received related to marital issues, grief or coming back from combat. Former investigators say that mental-health issues were particularly tricky to ferret out and understand in the context of a background check.

In a limited number of cases, candidates would sign waivers allowing USIS employees to obtain their mental-health records, these former employees say. USIS investigators said they then had to drive to the medical facilities where the candidates were treated and get someone to sign a form attesting to whether the candidate posed a threat to national security. The person who signed the note, however, could be anyone who had access to the candidate’s files; it didn’t have to be the person who treated the candidate, these employees said.

The bosses simply trusted the employees to do the work in the right way with little oversight, these former employees said.

“I could go weeks and weeks and not see a single co-worker, so there’s no way they can see what you’re doing,” said one former investigator.

There was a rule that all documents had to have two layers of locks: for instance, a locked file cabinet in a home, plus a locked front door would qualify. But former investigators said that aside from their initial training, no one went to check their homes to make sure the documents containing personal information were secure.

“People were leaving their laptops at Starbucks,” said one former investigator. “People were leaving cases on top of their cars, information blowing off. We had a lady that left her files at Chuck E. Cheese with her kids.”

A number of employees in the background-checking business have been convicted or have pleaded guilty to falsifying records. Several worked for USIS but at least one had been employed by a USIS competitor, CACI International.

“They’re going to make the numbers one way or another,” Privetera said. “Obviously, they’re not going to say, ‘We encourage bad behavior.’ But you’re kind of creating the environment for it.”

Matea Gold, Zachary A. Goldfarb, Tom Hamburger and Alice Crites contributed to this report.