NORFOLK, FEB. 4 -- Six pipe bombs were found attached to two giant chemical storage tanks here today, forcing police to evacuate thousands of residents in this port city that is one of America's largest military centers.

The bombs were discovered five miles from the world's biggest naval base, Norfolk Naval Base, and within miles of the Hampton Roads area's dozen other military installations, which together have sent about 40,000 service men and women to Operation Desert Storm.

Authorities said no group had taken responsibility for planting the bombs. Bill Baker, assistant FBI director for criminal investigations, said it was too early to tell whether the incident might have been related to the Persian Gulf War, but he added that "we have to keep that door open." So far, there have been no confirmed reports of attempted terrorism in the United States since the war began last month.

Norfolk Mayor Joseph A. Leafe said authorities have no suspects or motives. "We have no indication this is in any way related to what's going on in the Persian Gulf, nor any evidence it is not," he said. Officials noted that there were other possible explanations, such as a vendetta against the tanks' owner.

Workers at nearby businesses and residents of the blue-collar Berkley neighborhood within a half-mile of the tanks were evacuated about 8:30 a.m. Later, the evacuation area was extended to one mile. Police drove through the streets issuing warnings over loudspeakers, and public buses were used to collect those who needed rides. An evacuation center was set up at Booker T. Washington High School, but few residents used it. Residents were allowed to return home about 4:30 p.m.

"I was a little frightened," said Mary Freeman, who has lived in her home about a mile from the scene for 39 years. "If they had a war, I thought they were going to fight it in the air or the ocean, but I didn't know they would strike the neighborhoods where we're living."

Another resident of the neighborhood, Gloria Peace, said she saw a report about the bombs on television and ran out of her house to find out what was happening.

"I saw all the helicopters," she said. "I didn't know what was going on, and then when I did find out, I thought, 'Oh, my God, this close to home?' "

Cpl. Bob Haynes, a Norfolk police spokesman, would not describe the type of explosive contained in the pipes, nor would he speculate on why the timers were not ticking. But he said the bombs appeared to be made by someone familiar with explosives.

"You can pretty well determine that this was not an amateur that put these devices together," he said. "Were they supposed to go off or not go off? Were they supposed to go off hours down the road? That's what they're trying to determine."

The tanks where the bombs were planted do not serve the military. Except possibly for the nearby Colonna Shipyard, which is privately owned, authorities said there is no strategic military facility close enough to have suffered damage if the bombs had exploded. River and air traffic in the area were restricted most of the day.

Like Washington and other areas of the country, Hampton Roads has experienced an increase in the number of bomb threats since the war began, and security has been tightened at military bases, airports, government buildings and private companies. However, until today, no real bombs had been found.

The incident began about 7:45 a.m., when a worker at Allied Terminals Inc., a chemical storage facility with about 15 tanks in an industrial area south across the Elizabeth River from downtown Norfolk, found the first bomb. It was attached to a valve on an above-ground tank containing 1 million gallons of methanol, a highly flammable liquid used as a solvent and in antifreeze.

About 10 a.m., Virginia State Police bomb technicians outfitted in protective suits went in to study, videotape and X-ray the device, whose timer was not ticking.

During a search of the storage facility, police found a second bomb attached to a valve on another tank about 150 feet away. The tank contained an undetermined amount of sodium sulfide, which is used in fertilizer and detergent. It is not highly explosive, but can release a poisonous gas when heated.

The first bomb was made up of a pair of two-foot-long pipe bombs linked by a detonator-timer. Technicians carried the bomb a short distance away. Using a small controlled explosive, they blew off the caps at both ends of the pipes to relieve any internal pressure, effectively disarming it.

"They want to be able to save any evidence they can," Haynes said.

The second bomb, made up of four similar pipe bombs with a single timer that also was not operating, was taken to a cemetery, buried under earth by bulldozers and detonated about nine hours after the incident began.

About noon, someone called Norfolk police to report that another bomb had been placed at a storage tank at nearby Miller Oil, but after an exhaustive search, city fire department officials reported that no device was found. They concluded that the call probably was a hoax.

How much harm an explosion at the methanol tank might have done would have depended on what kind of damage was done to the tank, experts said. If the bomb punched a hole in the tank where the methanol liquid was located, it could produce only a leak, they said. But if the bomb had exploded in the vapor space between the surface of the methanol and the top of the tank, a large blast could have resulted.

"You get a big boom and the cups rattle" in nearby homes, but the explosion would not have left a mile-wide cataclysm, said William J. Cruice, vice president of Hazards Research Corp. of Mount Arlington, N.J., a consulting firm.

Methanol, or wood alcohol, is a colorless liquid that is highly flammable, chemical experts said, potentially producing 50-foot flames.

"It can create a significant, significant fire hazard," said Fairfax County Fire Chief Warren E. Isman.

The chemical in the other tank, sodium sulfide, does not have much explosive potential, but potentially can release hydrogen sulfide, a colorless poison gas that smells like rotten eggs, experts said.

Frank M. Yeiser, medical director of the Norfolk Paramedical Rescue Service, said the chemical would not explode, but could have created a cloud causing irritation to the skin, eyes and lungs.

Even if the bombs prove not to be the work of terrorists, they underscored the anxiety in this area as the war goes on.

"If somebody's going to do something in the middle of Iowa, I don't think it gets their message across," said Robert Ivansen, a former Navy officer who lives in Virginia Beach. "But this is Atlantic Fleet headquarters, and there are many ships, Air Force squadrons and a lot of Marines from the area . . . . It's a very visible area."

Said Dora Perkins, one of those evacuated: "The first thing I said when the war broke out was the first place the terrorists would hit was Norfolk because of the naval base."

Staff writers John Ward Anderson, D'Vera Cohn, Stephen C. Fehr and George Lardner Jr. contributed to this report.