New York, San Francisco and Philadelphia are suing the Department of Defense over background checks for gun owners.

The three cities have all filed lawsuits, arguing that the department's lack of transparency has led to the loss of innocent lives.

The lawsuit accuses the Department of Defense of failing to report criminal convictions to the national criminal background check system.

New York City Mayor Bill de Blasio says the Department of Defense has "put guns in the hands of criminals."

According to the lawsuit, the US Air Force, Army and Navy all failed to submit records to the FBI's national background check system.