Washington ? Tens of thousands of federal airport screeners. Bulletproof cockpit doors. Closer scrutiny of ships and cargo.

There have been many improvements to transportation security since the Sept. 11 terrorist attacks but gaps remain, lawmakers said Tuesday. They cited security loopholes at the nation’s ports and the threat that a shoulder-fired missile could hit an airliner.

“Transportation security is at its highest level ever, particularly aviation security,” said Sen. John McCain, R-Ariz., and chairman of the Senate Commerce Committee, which heard from the Bush administration’s top transportation security officials. “However, we need to remain vigilant across all modes of transportation.”

Two months after the 2001 attacks, Congress created the Transportation Security Administration to protect aviation, shipping and transit. The new agency was given dozens of deadlines to meet, mostly dealing with air travel.

Many of those deadlines were met, including hiring passenger and baggage screeners and checking all bags for explosives.

But Peter Guerrero, director of physical infrastructure issues for the General Accounting Office, said much more needed to be done.

Guerrero, whose agency is the investigative arm for Congress, testified that it could cost hundreds of billions of dollars to secure the country’s entire transportation network.

“The magnitude of the problem here is almost beyond comprehension,” said Sen. Frank Lautenberg, D-N.J.

A key concern is that terrorists will use shoulder-fired missiles to shoot down an airliner. Though patrols of airport perimeters have increased, airport officials acknowledge planes could be vulnerable to such an attack.

Transportation Security Administration Chief James Loy said the administration has helped fund projects to make airport perimeters more secure, such as fences and access control systems.

Guerrero also pointed out that only a small amount of 12.5 million tons of cargo is inspected before it was shipped by air every year.

Loy said the agency was working on a more thorough cargo-screening plan. Loy also said an airline passenger prescreening program was being developed to measure the risk of every passenger who boards a flight in the United States.

Improvements in aviation security, though, may make other parts of the transportation system more tempting targets for terrorists, said Sen. John Breaux, D-La.

Breaux and others on the committee said they are concerned about the vulnerability of U.S. ports. Democrats chastised the Bush administration for failing to request funding for grants to improve port security, while planning to request $87 billion for Iraq and Afghanistan.