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, 2001, 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, checking all bags for explosives and requiring background checks for airport workers.

But Peter Guerrero, director of physical infrastructure issues for the General Accounting Office, said much more needs 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, which includes 3.9 million miles of roads, 600,000 bridges, 361 ports and more than 5,000 public-use airports.

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

Guerrero said more federal money is needed to protect the transportation system, and federal agencies need to better coordinate their efforts to eliminate duplication.

Ranking passengers

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 is shipped by air every year.

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

TSA spokesman Brian Turmail said the agency plans to give each passenger a ranking: those coded as terrorists or violent fugitives will be turned over to law enforcement; high-risk passengers will be given additional screening; and those who are cleared will go through routine screening. The vast majority of passengers will fall into the latter category.

The prescreening program, which will match a traveler's name, birth date, address and phone number with information from commercial and government databases, will be tested for the next six months before it is implemented.

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.

"More money to protect the ports in Iraq than here," Boxer said.

Coast Guard Commandant Thomas Collins said the administration requested a 30 percent increase in his budget for next year. Armed Coast Guard officers now board suspicious ships before they enter a port, and require oceangoing vessels approaching the United States to give 96 hours advance notice, among other efforts.

Collins said new rules will require some ships to install a transponder so the Coast Guard can tell where they are, but conceded the towers to receive the signals haven't been built yet.