Contractors could take over passenger screening at more airports under legislation approved by Congress and signed by President Obama.

The long-delayed authorization bill for the Federal Aviation Administration overrules the Transportation Security Administration’s decision to stop approving private screeners. TSA Administrator John Pistole said he did not believe contractors offered “clear and substantial” advantages.

The law reopens a debate that seemed settled after the 9/11 terrorist attacks, when Congress turned airport screening over to federal employees. Critics charged that poorly trained contract screeners allowed terrorists to carry weapons aboard the planes that crashed into the World Trade Center towers, the Pentagon and a field in Pennsylvania.

Sixteen of the nation’s 450 commercial airports have opted to hire contractors supervised by TSA, but most of them are smaller facilities. No airports had been allowed to privatize screeners since 2010 until this month, when TSA approved a contractor at the small West Yellowstone Airport in Montana, which has commercial flights only during the summer tourist season.

TSA’s screeners have come under criticism for long delays in security lines and intrusive searches of passengers.

Under the law, TSA must approve an airport’s application to privatize screeners unless it determines that privatization would not be cost-effective or would harm security. The agency must provide an explanation whenever it rejects an airport’s application to switch to contractors.

Contract screeners have to meet the same qualifications as TSA employees and must be paid at least equally. TSA has authority to approve contractors at each airport and monitor them.

Managers of several large airports have told Congress they want to hire contractors. Mark VanLoh, director of the Kansas City airport, told the newspaper Federal Times that employing contract screeners allows him to add additional workers on short notice without waiting for approval from Washington.