PHILADELPHIA (AP) — A crew member refusing to talk to federal investigators about a fatal boat crash in Philadelphia was piloting the tugboat pushing a barge that slammed into the duck boat, a Coast Guard official said Monday.

The mate exercised his Fifth Amendment constitutional right against self-incrimination and refused to meet with investigators over the weekend, according to the National Transportation Safety Board.

US Coast Guard Capt. Todd Gatlin confirmed to The Associated Press that the mate was on duty as the tug pushed a 250-foot barge up the Delaware River. "Duck boats" are civilian versions of the DUKW amphibious vehicle of the Second World War.

The crew of the duck boat told the NTSB that its radio calls to the tug "received no response," although other boat operators nearby reported hearing them.

The collision last week sank the tourist vessel, dumping 37 people overboard and killing two young Hungarians.

The tug, The Caribbean Sea, had been moved to Philadelphia on June 24, Gatlin said.

It previously had been in New York Harbor, according to Joseph Dady, a national tug safety advocate who once piloted the vessel.

The tug's crew consisted of a captain, the mate, an engineer and two deckhands, the NTSB said. "The mate was on duty ... and the captain was off," Gatlin told the AP.

By law, either the captain or mate must be at the wheel at all times, said Dady, president of the National Mariners Association and a member of the Coast Guard's Towing Safety Advisory Commission.

An 18-year-old trainee had been at the wheel of the duck boat when it entered the water, but the captain took over when the engine appeared to smoke, a passenger said Monday. The pair cut the engine, dropped anchor and were waiting calmly for help for several minutes when they saw the hulking barge bear down on them.

"Our younger fellow was out there flailing and calling, and obviously nobody saw him. I came to find out that nobody was on deck on the barge," passenger Sandy Cohen said Monday from her home in Durham, North Carolina, "And then they couldn't reach them by radio."

The tug's owner, K-Sea Transportation Partners of East Brunswick, New Jersey, declined to identify the mate or describe the crew's experience level.

Nor would the company say if there was a lookout on the barge, which Dady said is required if the pilot's view from the wheelhouse is significantly obstructed.

K-Sea has provided legal counsel to the five-person crew, but a spokesman could not immediately name the mate's lawyer.

The company said it was cooperating fully with the probe.

"If an individual chooses to take the Fifth Amendment, that's fully their right," spokesman Darrell Wilson said.

The captain submitted to an NTSB interview, but NTSB spokesman Keith Holloway would not disclose what he said.

Typically, tug captains and mates rotate six-hour shifts, with one person on duty and the other on break, Dady said. The deckhands also rotate shifts, and the NTSB said one was asleep at the time.

"It's 90 percent boredom and ten percent sheer terror," Dady said of a tug captain's job.

The amphibious duck boats are a popular way for tourists to see the sights of Philadelphia from both land and water.

Two Hungarians visiting the city as part of a language program, 20-year-old Szabolcs Prem and 16-year-old Dora Schwendtner, were missing for two days before their bodies were found.

Ten passengers suffered minor injuries.

The tug was pushing a city-owned barge that carries sludge a few miles downriver to a wastewater treatment plant.

The barge — empty and riding high on the sea — was making the return trip upriver when it struck the tourist boat about 150 feet (om the shoreline, where commercial, tourist and pleasure craft share space in the Delaware River's deep shipping channel.