About 83,000 Pentagon employees and contractors who held or were eligible for secret clearances had more than $730 million in unpaid taxes as of June 2012, according to an internal government audit.
"Federal laws do not prohibit an individual with unpaid federal taxes from holding a security clearance, but delinquent tax debt poses a potential vulnerability," said the Government Accountability Office, which conducted the report.
The report underscores concerns in the government that individuals with access to classified information could become targets of foreign intelligence agents if they are having financial troubles.
It also comes as the government is trying to crack down on so-called insider threats similar to NSA contractor Edward Snowden, who stole millions of classified documents before leaking them to the media and seeking asylum in Russia.
The GAO report found more than 5.1 million civilian and military employees and contractors held a security clearance as of October.
Of the 83,000 Pentagon employees and contractors with both delinquent tax debts and security clearances, about 26,000 actually had access to secret documents at the time of the study, which was conducted from 2006 to 2011. Combined, those employees owed about $229 million in back taxes, GAO found.
About 4,800 of the 83,000 employees had IRS liens against their property, and 23,000 were subject to wage garnishment and other IRS collection tactics.
The GAO first recommended in 2013 mechanisms to improve federal agencies' ability to detect delinquent federal tax debts by those eligible for security clearances.
However, statutory privacy protections limited access to this information, the GAO said. The Office of the Director of National Intelligence (ODNI) had formed an interagency working group to explore creating an automated process similar to what the Treasury Department uses.
In June, the ODNI's working group said due to the legal and logistical challenges of obtaining tax compliance information from Treasury, it was exploring other sources of information.
The working group said it was their goal to establish an automated system by 2017, but efforts are still in the initial planning stages.