A report issued by an analysts' group Monday calls for more government action to protect U.S. ports and ships from terrorists, as well as better planning to rapidly restore services in any area that is attacked. The Rand Corporation is a well-respected private research organization. Part of the challenge is developing new screening methods and technology.

Defense planners and counter-terrorism experts have long worried about the possible use of shipping containers to smuggle bombs or other weapons into the United States. They also worry about the detonation of a bomb on a ship, which could cripple a port facility, like the one here in Houston. The authorities here have taken extra precautions to prevent any action that could block the channel leading to the Gulf of Mexico or damage nearby refineries and fuel storage facilities.

Critics have noted that only around five percent of containers coming into U.S. ports are inspected. But Henry Willis, principal author of the Rand Corporation report, says inspectors are trying to keep a balance between security and the efficient operation of the $500 billion-a-year containerized shipping industry. He says inspectors have to rely on imperfect X-ray or gamma-ray scanning technology to check suspect containers.

"One of the problems with scanning is that it can miss things," he said. "Another problem is that it can identify containers as high risk when they are not. In fact, that is one of the problems that is driving the cost of scanning, in that you send a lot more containers over to hand inspection, which is a timely [time-consuming] and costly process."

Mr. Willis says what is urgently needed is a set of procedures, using newer technology, that can keep security costs low, while building a greater deterrent against terrorists.

"I do not think there is a single solution that is going to solve this problem," he said. "What we really need is a portfolio of approaches, a gauntlet, that people who want to try to attack us using the system have to get through."

Terrorists could employ methods already used by drug smugglers, sending in multiple shipments, assuming that some will be discovered, but that others will not. But Henry Willis says improved inspections would reduce the benefits of such an approach. And the bigger the weapons involved, the more likely they would be to be discovered, and the higher the cost would be for the terrorists in terms of lost money and lost logistical secrets.

"Terrorism is, in some ways, a two-player game, and we have to think about what the costs to a terrorist are of a failed attempt," he said. "If the costs of a failed attempt are very low, they might try to send multiple bombs or devices through the system. If the costs are high, they are less likely to do that, and more likely to be deterred."

The Rand report also calls for better plans to deal with a disaster when and if it happens. Each day that a major port is closed down, Mr. Willis argues, adds to the benefit the terrorist achieves in the attack. Improved response and recovery would reduce such benefits.

The U.S. government has addressed the threat of terrorism to ports and shipping through a number of programs, including The Customs Trade Partnership Against Terrorism, also known as CTPAT, and through cooperative efforts with other nations under the U.S. Maritime Transporation Security Act. U.S. authorities say they are constantly searching for better technology and measures to protect both commerce and people from terrorism.