Commercial air travel is at risk from terrorists who quietly get jobs at airports so that they can attack from within sensitive areas, a senior Homeland Security Department official told lawmakers Wednesday.

There has never been such an instance, but a security supervisor at Newark Liberty Airport is facing criminal charges that nearly 20 years ago he assumed the identity of a New York man who was later murdered. This incident raised questions about whether the Transportation Security Administration knows the true identities of people who work in secure areas at airports around the country.

The TSA said the man, Nigerian Bimbo Olumuyiwa Oyewole, never worked for TSA and the agency did not issue the man's security badge. The TSA requires a background criminal and terror check for employees who work at airports. Oyewole was screened through that process, but because he worked at the airport for so long, the TSA did not do a separate check that would verify his identification, the agency said.

The House Homeland Security Committee conducted an oversight hearing Wednesday, and a senior TSA official, assistant administrator John Sammon, said he could not assure lawmakers there were no other such cases around the country.

"We don't know whether they are who they say they are," said Rep. Bennie Thompson, D-Miss.

People who pose a threat can obtain government security badges for U.S. airports because the TSA inadequately investigates the backgrounds of badge applicants, said the agency's acting inspector general, Charles Edwards. This includes missed signs that such people might be dangerous, or confirmation they are American citizens, Edwards said.

Some of these security gaps could be resolved with a new government rule that would require a criminal history check every five years and strengthen other identification reviews, Sammon said. But he acknowledged that he couldn't assure lawmakers that the new rules — still under development — would catch every case of an airport worker using someone else's identity.

Oyewole is charged with using the identity of Jerry Thomas, a petty criminal who was shot outside a Queens, N.Y., YMCA in July 1992. Thomas' murder remains unsolved, but police in New York said Tuesday there is no evidence tying Oyewole to Thomas' shooting.

However, two law enforcement officials with knowledge of the investigation said that Oyewole began using Thomas' birth certificate and Social Security number three weeks before Thomas' murder. The officials requested anonymity because they weren't authorized to discuss details of the case publicly.

Authorities were alerted to Oyewole's alleged double life when the Port Authority of New York and New Jersey's inspector general's office in Hoboken received an anonymous letter, the officials said. The letter described Oyewole's using additional names, though those weren't divulged Tuesday.

The Port Authority, which operates the area's main airports and other transit hubs, said Oyewole entered the United States illegally in 1989 and had worked under several contractors at the airport, most recently FJC Security Services, and had supervised about 30 guards. The agency said its investigation found no indication that he used the fake identity for any reason other than to live in the United States.