Even though the amphibious vehicle known as "Duck 34" wallowed powerless in the Delaware River for about 15 minutes before its fatal sinking, no distress call was ever received by the Coast Guard, investigators said Thursday.

The Wednesday afternoon crash that sent 37 passengers into the water - and apparently killed two - has left investigators with a puzzle: why did two vessels equipped with radios, required to keep a look out, and crewed by licensed mariners, collide in clear weather on a sunny day?

Some answers may be known Friday, when the national Transportation Safety Board plans to interview the tug boat's five-person crew, and the captain and deck hand who were on board Duck 34. It was struck by a 250-foot long barge being pushed by the tug.

The NTSB will have to determine if proper radio warnings were broadcast by Duck 34, and whether the crew aboard the tug "Caribbean Sea" was monitoring marine radio and keeping a lookout as it steamed up river, and pushing before it a city-owned, 250-foot barge called the "Resource."

A Duck 34 passenger, Alysia Petchulat, 31, was on board with her 12-year-old son and said captain Gary Fox initially used his radio to call the Ride the Ducks office to arrange for a tow.

When he spotted the barge, she said Fox, "tried to call three or four times," when he realized it was bearing down on his immobile vessel.

Marine radio broadcasts can be heard by anyone with a radio tuned to an identical channel. If Fox followed Coast Guard rules, he would have used channel 13, which the tug was required to monitor.

As Petchulat, a tourist from Missouri, recalled the scene, "He (Fox) said stop. . .we are anchored down and we cannot move. . .We are right here, please see us."

"They never responded," Petchulat said of the tug.

Meanwhile details on the missing passengers, Hungarian tourists, have started to emerge. The Coast Guard identified them as Dora Schwendtner, 16, and Szablcs Prem, 20. The Coast Guard officially halted its search for them Thursday at 6:55 p.m.

Anna Gyulai Gaál, a reporter for the national newspaper Bors, said that the two were from Mosonmagyaróvár, a town of 33,000 close to the Hungarian-Austrian border.

A group of 13 students from the local high school accompanied by two teachers arrived in the United States on July 2, Gaál said, and were scheduled to return on July 23. The trip was arranged through the Marshallton United Methodist Church in West Chester.

Marshallton was linked to the Hungarian youth via a Dutch Christian organization that connects young people from around the world.

Duck 34's final journey started around 1:30 p.m. near the Constitution Center. It entered the river on a ramp at the end of Race Street, and headed south near the shoreline until its engine died shortly before 2:30 p.m.

A captain with Ride the Ducks, Harry Burkhardt, said his 18-year-old son, Kyle, was a deck hand on Duck 34 Wednesday.

Kyle Burkhardt told his father that in the minutes before the collision he and Fox made sure there was no threat of fire on board and that passengers were in their life jackets.

They also dropped the boat's anchor and alerted other Duck tour boat captains that they needed a tow.

Then, the elder Burkhardt said, his son told him he looked up at the looming barge and dove off the front deck seconds before the collision.

Kyle told his father that the "The boat was twisted in the collision; you could hear the metal crunching."

The accounts of survivors, and photographs of the impact, show Duck 34 partially submerged under the barge's high steel bow.

The photographs indicate the barge was moving slowly - there is little bow wave apparent, which would be easily visible if the barge was moving at high speed. After the collision the barge does not appear to have continued traveling upriver.

Standard procedures would have been for the Duck 34 crew to use channel 13 to warn nearby shipping that it had lost power; the crew also had the option of alerting the Coast Guard on channel 16, specifically designated for marine emergencies. Transmissions on that channel are automatically recorded.

Coast Guard Lt. Commander Matthew Rice said Thursday there is no recording of a may-day call on Channel 16 from either vessel.

Transmission on channel 13 is not recorded, and Rice said the Coast Guard only learned of Duck 34's breakdown in calls that came in after the collision.

Robert Sumwalt, an NTSB board member, said the navigational and communication gear on the tug will be checked, and that crew members on both vessels will be checked for drugs. Alcohol tests administered were negative.

The NTSB will also likely attempt to determine if the engine problems on Duck 34 could have prevented radio transmissions.

Passengers on Duck 34 will also be interviewed.

Mike Hanson, a spokesman for tug firm, K-Sea Transportation Partners L.L.C., said the company was cooperating with the NTSB investigation, but would have no comment on the circumstances of the accident.

The sinking was the first on-water incident involving Ride the Ducks since it began operations in 2003 in Philadelphia and it prompted the firm to shut down operations across the country. The company operates 15 ducks in Philadelphia, and designates each with a number.

Chris Herschend, president of Ride the Ducks, said, "We believe the operator followed all protocols including making a radio call." Company policy would have been for another boat to head out and assist with a tow, he said.

Herschend could not clarify what caused the power loss. "I know there was an overheating, we don't know it was a fire," he said.

The vessel was built in 2003 to resemble a World War II era DUKW that used to transport troops, a company spokeswoman said.

Herschend said each vessel is inspected twice every day by company maintenance workers, once before they are put in the water and again at the end of the day. They are inspected annually by the U.S. Coast Guard, and subject to additional inspections when problems occur.

There are no records of any problems with the boat that broke down, and Herschend said that "to his knowledge," the boat that broke down had no problems on Wednesday morning.

The company said one duck boat overheated on July 1 while it was onshore, and three others had "symptoms" of overheating this season.

A 1999 accident killed 13 people when a World War II-era duck boat sank on Lake Hamilton in Arkansas. The NTSB suggested a variety of modifications to the 1940s boats. Sumwalt said he did not know whether the newer duck boats in Philadelphia met those safety recommendations.

"That's something we're going to be looking at," he said. In general, he said, duck boat companies have not been responsive to the safety upgrades suggested by the NTSB in that report.

"Any time our recommendations are not acted upon, we have concerns about that," he said.

Herschend said he does not believe there are widespread problems with the company's boats. He described mechanical problems as "rare."

Duck 34 is expected to be raised Friday.