It does not have enough equipment to test how well screeners can identify threatening items carried by travelers themselves or in their luggage.
In addition, only 18 or 20 airports will get federal money to permanently install machines that detect bombs in checked baggage.
All those weaknesses are the result of budget problems, the federal official in charge of airport security told lawmakers Thursday.
But James Loy, head of the Transportation Security Administration, declined to tell the House Transportation and Infrastructure's aviation subcommittee how much more he thinks the agency needs than the $5.2 billion Congress has set aside this year. That figure includes money for federal air marshals.
"We cannot provide world-class, effective security on the cheap," Loy said.
Rep. Peter DeFazio of Oregon, the top Democrat on the subcommittee, said Loy had to come clean with the agency's budget needs or withstand criticism for its failures.
"We need to know honestly what you need," DeFazio told Loy. "If you need more money or people or expertise, technology, investment — you've got to tell the administration or us."
During the hearing, Loy acknowledged that explosives cannot be identified by the airport metal detectors that travelers walk through or by the X-ray machines that examine carry-on luggage. He said $60 million for developing equipment that could find bombs on air travelers had to be spent on other needs.
The subcommittee chairman, Rep. John Mica, R-Fla., said he was disappointed the money went to salaries.
"We are not one iota closer to routinely screening passengers and carry-on baggage for explosives," Mica said.
Lawmakers noted long security lines had returned to some airports and they questioned whether the agency has enough workers. The agency has cut 6,000 airport screener jobs since April after Congress ordered the reductions.
Loy said the agency has enough screeners — 48,000 now — and plans to have the equivalent of 45,000 full-time employees by Oct. 1.
Rep. Hal Rogers, R-Ky., led the effort to reduce the number of screeners.
"Some people think we ought to spend every cent we can beg, borrow or steal in the name of homeland security," said Rogers, chairman of the House Appropriations' subcommittee on homeland security.
Rogers praised Loy as an effective administrator trying to do an impossible job. But he questioned why the agency could only give 18 or 20 airports money to permanently install the bomb-screening machines because Congress had appropriated $1.9 billion to do so since Sept. 11.
Having that equipment in place saves time and could free up money for other security measures because screeners do not have to load luggage into the machines by hand.
Congress and the White House are focusing on the possibility of terrorists using portable rocket launchers — known as MANPADs, or Man Portable Air Defense Systems — to shoot down planes.
Loy told the committee that the intelligence he received indicated the rocket launchers pose little risk in the United States.
"There are no immediate threats with respect to MANPADs today," Loy said.