The Department of Homeland Security is catching less than 1 percent of people whose U.S. entry visas have expired, according to a new audit.

The Office of Inspector General at DHS estimated in its report that there were 527,127 nonimmigrant visa overstays in 2015. Of those, officers and Immigration and Customs Enforcement agents only arrested 3,402 — or only one for every 150 cases.

The inspector general added that there is a backlog of 1.2 million expired visa cases.

According to the IG, one problem is that the agency's technology used to "identify nonimmigrant visitor overstays" is flawed and not working properly. In order to figure out if someone left the U.S. when he or she was supposed to, officers have to use 27 different computer systems.

Even after doing that, the IG found that the data being used was often incomplete, leading to false reports that people are still in the country when they are not, or that they have left when they are still present. Officers also spent unnecessary time tracking down these false reports.

"Such false departure information resulted in [Enforcement and Removal Operations] officers closing visa overstay investigations of dangerous individuals, such as suspected criminals, who were actually still in the United States and could pose a threat to national security," the investigators wrote in the audit. "For example, [a deportation] officer stated that a suspect under investigation was listed as having left the country, but had given his ticket to a family member and was still residing in the United States."

Two of the Sept. 11 hijackers were overstays, which prompted the 9/11 Commission to make the federal government track all visitors in and out of the U.S.