This frame grab made from video released by CBS3 shows rescuers assisting passengers who were aboard a sightseeing boat that was knocked over by an oncoming barge in the Delaware River on Wednesday in Philadelphia.

Philadelphia ? An amphibious sightseeing boat that stalled in the Delaware River was knocked over by an oncoming barge Wednesday, spilling 37 people overboard and leaving two passengers unaccounted for after a frantic rescue effort.

Ten people were sent to a hospital 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,” police Deputy Commissioner Richard Ross said.

The search was scaled back to two boats at nightfall from nearly 20 boats earlier, along with a Coast Guard helicopter.

“Right now we’re at a point where, I’ll be honest, hope is fading, but we’ve not given up hope completely,” said Coast Guard Capt. Todd Gatlin, deputy sector commander in Philadelphia.

He said divers were able to reach the duck boat but could not get inside to see whether the missing people were there.

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.

Gatlin said there was no recording of a mayday call from the boat.

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 today at the earliest.

Late Wednesday, Robert Sumwalt, a board member of the National Transportation Safety Board, said the agency would investigate the sinking, including trying to figure out why the two vessels collided and “how conspicuous would that duck have been” to the tugboat that was pushing the 250-foot-long barge.

Gatlin said the duck boats are not allowed to go more than 100 yards out into the river. As far as the barge’s ability to avoid a collision, he said, “People don’t realize how tough it is to stop on the water.”

There were 35 passengers and two crew members aboard the duck boat, said Coast Guard Senior Chief Bud Holden.