Transportation Security Administration investigators only conduct risk assessments of about 40 airports over a three-year period, according to a federal audit, which also shows that the agency has failed to search for possible terrorists working at the airports.

Thirty-four airports deemed “high-risk” receive a triennial “joint vulnerability assessment” conducted by the TSA and the FBI. The assessments are one of two measures used to assess the vulnerability of airports, the Government Accountability Office said, but few other airports are reviewed as thoroughly. The TSA has conducted the assessments at four to six other airports each year but has not been able to conduct them at all U.S. airports, the watchdog said in a report released Tuesday.