While rescue crews in Philadelphia suspended their search in the Delaware River for two young Hungarian tourists lost in Wednesday’s amphibious boat crash, federal investigators focused Thursday on why no distress call had been recorded from the small tourist boat when it stalled on the water minutes before it was gorged by a large passing barge.

The crash threw 37 people from the “duck boat” into the river, leaving many to bob in life jackets fighting the current as the boat sank. By Thursday, the boat had settled on its six wheels some 50 feet below the surface; a crane was being prepared to haul it up from the depths for clues.

A Coast Guard spokeswoman, Petty Officer Crystal Kneen, said that rescuers suspended the search for missing tourists — a 16-year-old girl and a 20-year-old man —Thursday evening.

In Philadelphia and its suburbs, and as far away as the city of Mosonmagyaróvár, in western Hungary, the accident stunned government officials and a close-knit church community. Marshallton United Methodist Church in West Chester, Pa., was hosting 15 members of a Hungarian high school youth group for a three-week religious and cultural exchange program that started last Friday.

Seven youth group members and leaders from Marshallton were also on the boat and survived the crash, according to a statement on the church Web site. A prayer vigil was to be held Thursday night for the two missing passengers. Clues were scant on Thursday as members of the National Transportation Safety Board began speaking to crew members from the tourist boat, operated by Ride the Ducks tours, and from the tugboat that had been towing the barge owned by the City of Philadelphia.

Robert Sumwalt, a board member involved in the investigation, said that preliminary blood-alcohol tests from crew members — two from the duck boat and five from the tugboat — “all came back negative.” Drug test results, he said, were still pending.

The accident occurred around 2:30 p.m. Wednesday, when the duck boat encountered mechanical difficulty shortly after entering the water for the second half of its amphibious tour of historic Philadelphia. The engine stalled and at least one passenger told the authorities that they were waiting for another vessel to pick them up in the busy waterway.

Meanwhile, an unmanned and unmotored barge, which the Coast Guard said Thursday was 250 feet long and towed by a tugboat, kept getting closer to the helpless craft, according to several witnesses on the shore and passengers. Investigators were trying to determine why the tugboat did not stop.

“We have heard from the Coast Guard that there is no recorded distress call,” Mr. Sumwalt said. He added that investigators would also be looking into the possibility that the engine might have overheated, causing it to stall.

On Thursday, Ride the Ducks, the Norcross, Ga., company that operates the Philadelphia duck boat tour, suspended operations in Philadelphia and at its five other locations around the country — Branson, Mo.; Newport, Ky.; San Francisco; Stone Mountain Park, Ga.; and Seattle, according to the company’s Web site.

Chris Herschen, president of Ride the Ducks, told reporters Thursday at a news conference in Philadelphia that the captain of the boat appeared to have followed all standard procedures. But Mr. Herschen would not say whether the captain had sent out a distress signal.

Michael Luo contributed reporting from Philadelphia.