Sitting with his 9-year-old daughter and dozens of other tourists in a disabled duck-tour vehicle, bobbing helplessly in the Delaware River, Kevin Grace watched as the hulking barge loomed close, on a collision course.

"We had 45 seconds to try to get the life jackets on our kids," he said Wednesday evening. Grace, a tourist from St. Louis, grabbed his daughter, but the next thing he knew, "it hit."

The crash capsized and sank the duck, a popular and ubiquitous Philadelphia tourist attraction, dumping 35 passengers and two crew members into the river near Penn's Landing.

After a frantic rescue effort, 35 people were plucked from the water, but two passengers remained missing late Wednesday - a 16-year-old girl and a 20-year-old man, both tourists from Hungary.

"We are actively searching for the two unaccounted-for individuals," Mayor Nutter said from the riverfront. "We are putting all of our effort and forces into that on the water, in the air."

The search was called off late Wednesday night and will resume Thursday morning. A Coast Guard ship was stationed near the site of the wreck.

The sunken vehicle was found at a depth of 40 feet using sonar, and police divers entered the murky water about 5:30 p.m., three hours after the accident. The divers could not tell if the missing passengers were still inside.

"You can't see three inches in front of you," said Philadelphia Police Commissioner Charles H. Ramsey.

The amphibious tourist vehicle - operated by Ride the Ducks - launched just south of the Benjamin Franklin Bridge before mechanical difficulties and a fire forced it to shut down, said Lt. Frank Vanore, a police spokesman.

"The boat was sitting on the water waiting for help," he said.

A city-owned barge, being pushed upriver by a private tug company, hit it.

The National Transportation Safety Board and the Coast Guard were investigating, authorities said.

The barge, the Resource, hauls sludge from the city's Northeast Wastewater Treatment Plant to the recently privatized sludge plant in Southwest Philadelphia run by Philadelphia Biosolids Services.

The barge, which is unmanned and unmotored, was empty. The city has a contract with K-Sea Transportation Partner L.L.C., which operates the tugboat that was pushing the barge.

A spokesman for the company, headquartered in East Brunswick, N.J., said that the tug - the Caribbean Sea - had a full crew of five and that the captain and first mate had proper Coast Guard licenses. The captain called the company to report the accident, said the spokesman, Darrel Wilson. He said he knew few details.

"At this point we're not even concerned about who is at fault," Wilson said. "We're just anxiously waiting to hear that everyone is fully accounted for."

Capt. Stuart Griffin, a pilot and officer of the Pilots' Association for the Bay and River Delaware, said the tug was secured to the barge "at the hip," on the rear, left side.

Griffin said that he had no knowledge of this accident, but that if the tourists had been to the right of the barge, "the barge could have obstructed the line of sight. . . . I'm guessing he didn't see them."

Late Wednesday, Coast Guard Capt. Todd Gatlin said his investigators had no sense that the tug boat captain saw the duck boat.

The Coast Guard monitors and records Channel 13 as well as Channel 16, the emergency and hailing radio channel. Gatlin said investigators reviewed the 45 minutes before the accident and shortly thereafter. From the Channel 16 recording, Gatlin said, there was only the sounds of the "scramble" in the final moments before impact, but "the final recording was not a call for help."

The Coast Guard also had a report from the captain of another vessel, who heard some radio discussion that might have been between the duck boat hailing another ship. Gatlin said the duck boat captain had turned the radio on and off while dealing with the engine problems and smoke.

Passengers board the ducks at Independence Mall. They tour Old City before entering the river at a ramp. The accident happened near the ramp, in view of many witnesses along the riverfront.

As the two vessels drew close, Jason Tilghman said, he heard a duck crew member scream to the passengers, "Jump!" Just before the duck was hit, the crew member leaped into the water, followed by four passengers, Tilghman said.

"Everyone panicked, rushing to the front of the boat," said Grace, 50, who was on the tour with his daughter, a 31-year-old friend, and the friend's 11-year-old child.

Frederick Landwher, who watched the accident from Adventure Aquarium in Camden, said the duck vehicle had rolled over and its roof collapsed before it disappeared under the barge.

"You see all the life jackets pop up, and then you see all the people pop up," he said.

A hodgepodge of boats scrambled to rescue the survivors, including a team of Navy SEALs who were in town with their boats for a ceremony. Philadelphia police officers dived into the water. Regular citizens grabbed fire hoses and ropes to toss as lifelines for the passengers.

Grace said that he had tried everything he could to hold on to his daughter, but that she had slipped from his grasp when the duck rolled over.

"In the frenzy," he said, "I managed to grab a cooler." Struggling against the current, he tried to use the cooler to keep his daughter from being swept away.

Talmadge Robinson, a nurse's assistant who had gone to Penn's Landing after work, said he helped pull three children in life jackets out of the water.

"I looked out and all of these kids were in the river," he said. "They were pretty scared. All they could say was, 'Thank you.' "

He said the children had slowly drifted within reach after about 10 minutes, but he saw police rescue other children who were not wearing life vests.

"I think we had one life jacket between us," Grace said.

A Pennsylvania rule that requires children 12 and younger to wear life jackets applies only to recreational vessels, said Chris Edmonston, director of boating safety at the Boat US Foundation.

He said commercial vessels must have enough life vests for everyone on board, but passengers are not required to wear them. He said he assumed the duck captain would have asked the passengers to don their jackets once the vessel lost power.

"That's what I would have done, but he is not legally required to have them do it," Edmonston said.

A large group of the tourists were from Hungary, including three children who were taken to Hahnemann University Hospital, a medic said. Ten people in all were taken there for treatment of mostly minor injuries, the Coast Guard said.

Along Penn's Landing Wednesday night, dozens of people sat on benches and watched as rescue and recovery crews searched the murky water for the sunken tour vehicle and the two missing people. Temple University student Joe Arinello arrived from his home in University City.

"I've never seen anything like this," he said. "I wanted to see how they did it."

Ride the Ducks, which began operating in Philadelphia in 2003, runs 15 vehicles in the city. The company was founded in 1977, and has about 90 vessels in several cities, including Atlanta, Baltimore, Memphis, and Branson, Mo.

Ride the Ducks is owned by Herschend Family Entertainment, which is based near Atlanta and operates Camden's Adventure Aquarium and the Dollywood theme park.

The ducks can carry 37 passengers and two crew members, said Sharla Feldscher, a local spokeswoman for Ride the Ducks.

"We're doing everything we can for the passengers and our guests," she said.

Ride the Ducks has canceled all tours for Thursday.

In 1999 a failed drivetrain seal caused the drowning deaths of 13 tourists, including three children, when a World War II military surplus amphibious vehicle sank in Lake Hamilton, Ark.

According to the Ride the Ducks website, the vehicles used in Philadelphia are "based on the classic WWII DUKW amphibious design," but "today we build our vehicles from the ground up using the latest in marine design and safety."

The vehicles are "regularly inspected, tested and certified by the United States Coast Guard," according to the company website.

About three hours after being rescued, Grace seemed remarkably calm. Asked to assess the experience, he said without hesitation, "Life affirming."