Ten people were sent to hospitals after the capsizing of the six-wheeled "duck boat," which offers tours of Philadelphia by water and land. Only minor injuries were reported. Witnesses said many passengers were wearing life vests as rescuers plucked them from the water.
Searchers spent hours looking for a 16-year-old girl and a 20-year-old man believed to have been aboard the vessel, police Lt. Frank Vanore said.
"It's remarkable that we're only looking for two people," Deputy Police Commissioner Richard Ross said.
The search was scaled back to two boats at nightfall.
The duck boat had driven into the water just after 2:30 p.m. and suffered a mechanical problem and a small fire, officials said. It was struck about 10 minutes later by a barge used to transport sludge, then sank.
Police Commissioner Charles Ramsey said divers found the duck boat in water about 50 feet deep. Vanore said crews would not attempt to recover it until Thursday at the earliest.
There were 35 passengers and two crew members aboard the boat, said Coast Guard Senior Chief Bud Holden. Coast Guard boats assisted by police and fire crews worked to rescue people from the water, he said. A spokeswoman for the duck boat company said 39 people were aboard, and the reason for the discrepancy wasn't clear.
"A barge went into us," one of the duck boat's passengers, Sandy Cohen, told WPVI-TV. "We had engine trouble, so we were just waiting for somebody else to come and tow us."
Bystanders along the waterfront screamed as the barge hit the boat, said a security guard who was patrolling the waterfront.
"I whirled around as the barge began to run over the duck boat," said Larry Waxmunski, a guard for the Delaware River Waterfront Corp. "After the barge hit it - it almost looked like slow motion - the duck boat began to turn over."
"Fortunately, you began to see the life vests popping up almost immediately," Waxmunski said. He then saw police boats beginning to pluck the tourists out of the water.
Television footage showed at least five people being pulled from the water wearing life vests in an area of the river near the Old City neighborhood, popular with tourists. Helicopter footage showed people in life vests being helped from boats on to a dock and at least one person on a gurney.
Terri Ronna, 45, of Oakland, N.J., said she was on a ferry going from Camden, N.J., across the river to Philadelphia when the captain announced that there was someone overboard from another ship and that they were going to rescue him.
"We were not even halfway over when they said there was somebody overboard and we were going to get them," Ronna said. "There were people all over; we could see all these orange life vests."
At a waterfront news conference, Mayor Michael Nutter said authorities were trying to figure out exactly what happened.
"This is a very serious situation, and we are going to do everything we can to get to the bottom of it," he said.
Hahnemann University Hospital spokeswoman Coleen Cannon said 10 people were taken there, but two refused treatment. The other eight, including two teens, two adults and three adults, were released Wednesday night.
The American Red Cross said members of a Hungarian church group were among the passengers aboard the capsized boat, but their names were not released.
One crew member from the duck boat was rescued by the ferry that the Delaware River Port Authority was operating on its scheduled route between Philadelphia and Camden, authority spokesman Ed Kasuba said.
Officials said the barge was owned by the city and being directed by a tugboat owned by K-Sea Transportation Partners of East Brunswick, N.J.
The city Water Department uses the barge to transport sludge from a sewage plant in northeast Philadelphia to a recycling plant down river, said Maura Kennedy, a Nutter spokeswoman. The city has a contract with K-Sea, which operates the tugboat that pulled the unmanned and unpowered barge.
The duck boat was operated by Ride the Ducks, which also operates tours in San Francisco, Seattle, Stone Mountain, Ga., Newport, R.I., and Branson, Mo.
Ride the Ducks has been in Philadelphia since 2003. Passengers board the duck boats at the Independence Mall and are driven on a tour of Old City. Afterward they ride into the Delaware River from a ramp south of the Ben Franklin Bridge.
The sites seen along the 70-minute voyage include Penn's Landing, Adventure Aquarium across the river in Camden, the Liberty Bell and Ben Franklin's gravesite. About 25 minutes of the journey are spent on the river and, according to its website, the top speed is around 7 mph.
A spokeswoman at the company's headquarters in Norcross, Ga., said it would issue a statement when it had more information. Sharla Feldscher, a local spokeswoman for Ride the Ducks, declined to provide any details about the crash. She said safety is the company's top priority.
Holden, of the Coast Guard, said the duck boats are inspected annually, but he did not know when the boat involved in Wednesday's crash was last inspected.
Another Coast Guard spokesman, Thomas Peck, said neither craft was in a wrong lane.
A duck boat sank at Hot Springs, Ark., on May 1, 1999, killing 13 of the 21 people aboard after its bilge pump failed. The National Transportation Safety Board blamed inadequate maintenance and recommended that duck boats have backup flotation devices.
In June 2002, four people were killed when an amphibious tour boat, the Lady Duck, sank in the Ottawa River near Canada's Parliament.
Some of the vehicles are amphibious military personnel carriers dating back to World War II that have been restored and reconditioned for peacetime use. Known by their original military acronym as DUKWs, they were first introduced in the tourism market in 1946 in the Wisconsin Dells, where about 120 of the vessels now operate.
As of 2000, there were more than 250 refurbished amphibious vehicles in service nationwide, according to the NTSB. The federal agency is investigating the crash.