This story was reported by Gloria Campisi, Stephanie Farr, Jan Ransome, Christine Olley, Catherine Lucey, Natalie Pompilio, Regina Medina, Jason Nark, Ronnie Polaneczky and Dafney Tales. It was written by Will Bunch.

THEY CAME to see Philadelphia from around the nation and the other side of the world.

They were families who brought their kids from St. Louis or North Carolina, or touring teens from Hungary, finding what seemed like a cool way to see Philly in the middle of the worst heat wave in years.

But roughly an hour after 37 people set out for a midafternoon Delaware River ride on the amphibious tour vehicle called the Duck boat, everything started going wrong.

There was an engine fire, and the Ride the Ducks boat stalled out. The watercraft drifted, powerless in the Delaware. The Center City skyline loomed behind them, hazy behind the waves of 103-degree heat, and crew members told the tourists to hang tight - a rescue boat was on the way.

One of the passengers called her husband to tell him what was going on, that the Duck boat was dead in the water - and then suddenly the husband heard a scream. The phone went dead.

It was 2:39 p.m.

On the riverbank near Penn's Landing, Meg Scharpf, a tourist from Phoenix, saw what the husband on the phone could not see - a massive 250-foot barge, pushed by a tugboat and seemingly headed right toward the listless watercraft.

At first, Scharpf said, everyone on the boat appeared from the shore to be relaxed, even laughing. The Arizonan thought to herself there was no way the barge could strike the tourist-packed vessel, but that is exactly what happened. Some on the shore were yelling frantically toward the tug: "There's a boat! There's a boat!"

"There was nothing they could do," Scharpf said. "It was the most horrible thing I've ever seen. Everybody seemed to disappear and then they all started popping up with their life vest on."

Except that many had just a few seconds to get those life vests on - and not everybody made it.

"I pushed my [9-year-old] daughter over the side of the boat and tried to get the life jacket on her and I grabbed her by her hair as the boat rolled over on top of us," said one of the passengers, Kevin Grace, 50, of St. Louis. "It was really chaotic. It was flailing bodies everywhere and water and air bubbles."

Not everyone was as fortunate as Grace and his daughter. Until last night, police divers searched the murky waters and the watercraft - which capsized and sank 50 feet to the river bottom - looking for two passengers not accounted for.

The two were reported to be young tourists from the Hungarian group - a 20-year-old man and a 16-year old girl.

The 35 other passengers and crew were plucked from the fast-moving Delaware current, some by helicopter and some grabbing onto a hose. Ten of the passengers - some visibly shaken - were rushed to Hahnemann University Hospital but none of their injuries were serious. Two declined treatment and the rest had been released by last night. The others were treated at Penn's Landing, many at the Independence Seaport Museum.

City officials said the massive barge that struck the tourist-laden Duck boat, the Resource, was owned by the city but operated by a contractor, K-Sea Transportation. The barge hauls sludge from a biofuels plant in Northeast Philadelphia to a recycling plant in Southwest Philly.

Ride the Ducks, owned by Herschend Family Entertainment Co., of Norcross, Ga., runs similar land-and-water tours across the country and said it had never had an accident of this type before.

Nevertheless, the apparent Philadelphia tragedy is certain to rekindle the debate over the safety of tours using the amphibious vehicles that were developed for World War II.

In 1999, a Duck boat operated by a different company sank rapidly in Lake Hamilton, Ark., as the overhead canopy trapped passengers, 13 of whom died.

Here in Philadelphia, a new Duck-boat disaster may raise fresh questions about the intersection of tourism and industry on the busy and sometimes crowded Delaware - an intersection that may have turned deadly in a matter of seconds yesterday.