WASHINGTON (AP) — The government’s sprawling system of background checks and security clearances is so unreliable it’s virtually impossible to adequately investigate the nearly 5 million Americans who have them and make sure they can be trusted with access to military and sensitive civilian buildings, an Associated Press review found.

Case after case has exposed problems for years, including recent instances when workers the government approved have been implicated in mass shootings, espionage and damaging disclosures of national secrets. In the latest violence, the Navy Yard gunman passed at least two background checks and kept his military security clearance despite serious red flags about violent incidents and psychological problems.