WASHINGTON (AP) — A U.S. senator is urging the Obama administration to tighten procedures after a congressional audit found security problems at companies using radioactive material.

In a letter Thursday to the Nuclear Regulatory Commission, Sen. Bob Casey said he was troubled that the Government Accountability Office found at least two cases in which employees with extensive criminal histories were granted unescorted access to high-risk radiobiological sources. One employee stole from the company and the other impersonated a radiography inspector, the GAO found.