WASHINGTON (AFP) - The agency in charge of US transportation security came under renewed fire on Tuesday as a report revealed that American airports had hired dozens of people with terror links.

The Transportation Security Agency (TSA) is already reeling after a recent Department of Homeland Security report found that investigators could sneak fake bombs and weaponry through security with a 95 per cent success rate.

Homeland Security Inspector General John Roth discussed before lawmakers his new report that found the TSA failed to detect at least 73 people with links to terrorism who were hired by US airports.

By law, the TSA had "limited oversight" over the hiring process and thus "lacked assurance that it properly vetted all credential applicants," the report said.

Roth said the law needed to be changed to give the agency access to all relevant information, including FBI terrorist watchlists.

Becky Roering, an assistant security director at the Minneapolis-St Paul International Airport, told the hearing that former "badged" employees had even gone to Syria to join the Islamic State in Iraq and Syria.

During the same hearing, a TSA official criticised a program to pre-check passengers either randomly or because they have been given "known" or "trusted traveller" status and are deemed to pose a low risk.

"TSA is handing out 'PreCheck' status like Halloween candy in an effort to expedite passengers as quickly as possible," Roering said.

More than a million people have signed up for the TSA's PreCheck program, and another seven million have been randomly chosen for expedited boarding checks.

In one case, a former member of an extremist organisation found himself randomly given access to the PreCheck line but a TSA worker recognised him and alerted his superior.

Roering also said TSA staff have low morale and work in a climate of fear and distrust.

Last week, Homeland Security head Jeh Johnson announced new measures to improve security screenings at American airports after investigators were able to smuggle mock explosives and weapons through checkpoints dozens of times.

Johnson reassigned Melvin Carraway, the TSA's acting head, and replaced him with another interim director, Mark Hatfield.