The officials who are responsible for safeguarding the nation's intelligence secrets are trying to figure out how to better vet millions of employees and contractors with security clearances, after auditors found that some of those workers owed more than three-quarters of a billion dollars in unpaid taxes.

About 83,000 employees and contractors at the Department of Defense owed more than $730 million in unpaid taxes, the Government Accountability Office reported last month. Last year, the agency reported that 8,400 executive-branch civilian employees and contractors owed $85 million.

Debt is a threat when it is held by those handling classified information, government officials say, because it could make them vulnerable to pressure of bribery from foreign intelligence services or other entities willing to pay for it.

Officials are working on a system that would allow agencies to check the IRS standing of applicants for security clearances, a process currently hampered by technological gaps and restricted by privacy protections.

Agencies that offer security clearances now rely largely on credit reports and applicants self-reporting their tax debts, GAO auditors found.

The effort to address such debt isn't new. Nor is the threat to national security. In the 1980s, several people were convicted of selling secrets after falling into debt, including NSA analyst Ronald W. Pelton and retired Navy officer John A. Walker Jr.

Now, officials are focusing more attention on threats posed by insiders, following the leak of classified documents by former National Security Agency contractor Edward Snowden.

Snowden appears to have been motivated by ideological, rather than financial, issues.

"Over the past year, the buzz phrase that everyone kept talking about was 'insider threat,'" said Evan Lesser, managing director of the career matchmaking site ClearanceJobs.com. "After Edward Snowden, I think people in government really took notice and understood that it's not necessarily external sources trying to hack into our classified systems that are the biggest threats. It's the people that we have that are accessing them every day and have some type of privileged access."

After the Snowden leaks, President Barack Obama and Congress ordered more funding to address insider threats. That included $75 million for a new computer monitoring program intended to catch an employee who was downloading information improperly.

"I hope we can continue to identify employees who are facing financial hardships before further problems arise," said Rep. C.A. Dutch Ruppersberger of Baltimore County, the top Democrat on the House Intelligence Committee. "Everyone must pay their taxes, including government workers who could become potential vulnerabilities when it comes to our national security."

The GAO raised concerns about tax debt last year when it reported on $85 million owed by civilian workers. Federal law doesn't prohibit people with debt from getting security clearances, but debt must be considered when clearance applications are decided.

Concerns grew last month when the GAO reported the $730 million in unpaid taxes owed by defense workers.

"It makes it all inherently vulnerable," said Seto Bagdoyan, acting director of the GAO's forensic audits and investigative service. "It's an indicator of vulnerability if a person carries a debt of any kind, especially after they get their clearance."

Sen. Tom Coburn of Oklahoma, the top Republican on the Senate Committee on Homeland Security and Government Affairs, expressed concern about that debt in light of rapid growth in the number of security clearances approved since the attacks of Sept. 11, 2001.

"It is vital that the Administration and Congress work diligently to eliminate potential threats that compromise the integrity of the federal workforce and the privileged information they safeguard," Coburn, one of the lawmakers who requested the GAO study, said last month. "Giving security clearances to individuals who fail to follow the law is unwise and risky. Federal tax cheats with security clearances jeopardize both our national and economic security, and could unnecessarily put our nation's classified information at risk."

Coburn said the country "must take prudent precautions not only to enhance our security, but also to encourage federal employees to pay their share of taxes and live by the same rules that so many hard working Americans do."

A Pentagon spokeswoman said the Defense Department supported the GAO audit and efforts to improve its awareness of tax debt among clearance holders.

But she also said that tax debt is and will continue to be just one of many factors considered in security clearance application reviews, which she said function around "whole person" assessments.

"All available, reliable information about the person, past and present, favorable and unfavorable, is considered in reaching a determination" as to whether the granting of a clearance is "consistent with the interests of national security," Navy Cmdr. Amy Derrick-Frost said. "Each case must be judged on its own merits."