FBI agents have been contacting hundreds of dive shops around the country out of concern the next wave of terrorist attacks could be carried out by scuba divers.

The FBI said it is looking into whether Al Qaeda operatives have been taking scuba training in order to blow up ships at anchor, power plants, bridges, depots or other waterfront targets.

Agents spent several days last week at Ocean Enterprises, one of the biggest dive shops in the country, checking customer files and sales of highly specialized scuba equipment, according to owner Werner Kurn.

"They want to know if we have seen anything out of the ordinary," said Kurn, whose shop is a few blocks away from an apartment complex where two of the Sept. 11 hijackers lived in 2000. "If you ask me where's the best place to dive, that's normal. If you ask me in a limited-visibility dive how do you maintain your bearings or how can I dive in the harbor, that's not."

The possibility that members of Usama bin Laden's terrorist network might have taken scuba training emerged from interrogations of people taken into custody in the U.S.-led effort to root out terrorism around the world, said John A. Sylvester, who heads the counterterrorism office in the FBI's San Diego bureau.

A warning of possible attacks by divers was issued by the government before Memorial Day. The Coast Guard also warned of the possibility last weekend, and security around ports and ships has been tightened as a result.

Last week, the Professional Association of Diving Instructors, the world's leading diving organization, gave the FBI a list of 2 million people the association has certified to dive over the past three years, Vice President Jeff Nadler said.

Agents are contacting all U.S. dive shops to check the names of those who took scuba courses over the past three years, including those who dropped out without getting certified, Sylvester said. The United States has about 1,200 dive jobs, Nadler said.

Also on the checklist are U.S. commercial dive schools that train students in underwater welding and repair.

At Ocean Enterprises, agents have checked the infrequent sales of $5,000 rebreathers, devices that allow Navy SEALs to swim without producing a telltale trail of bubbles. Also of interest were sales of underwater propulsion vehicles that can tug a diver long distances and sell for as much as $8,000 each, Kurn said.

In addition to California, dive shops in the Pacific Northwest, Florida and Ohio said they have been contacted. Dive shops in landlocked states, like Scuba One in Mandan, N.D., also are getting calls.

The FBI said San Diego is a crucial part of the investigation. It is both a dive center and a major tourist destination. The area is home to a host of potential targets, from seaside nuclear plants to cruise ships and nuclear-powered Navy submarines and aircraft carriers.

Some scuba experts said they are doubtful that a diver could do little more than make mischief underwater.

"Is it feasible? It sure it is," Nadler said. "Is it likely? It'd be kind of tough."

Navy SEALs — frogmen who learn to attach limpet mines that can blast through ships' steel-plate hulls — must train for years before they are certified as combat swimmers skilled in underwater demolition.

"It's well beyond the skill level of a scuba diver," said Master Chief William Guild of the Naval Special Warfare Center in Coronado, near San Diego.

While scuba schools train people to dive in clear conditions with good visibility, the water around bridges and ports is turbulent and cloudy.