PHILADELPHIA – A crew member who is refusing to talk to federal investigators about a fatal duck 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 right against self-incrimination and refused to meet with investigators over the weekend, according to the National Transportation and Safety Board.

Coast Guard Capt. Todd Gatlin confirmed that the mate was on duty as the tug pushed a 250-foot barge up the Delaware River. 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 said.

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 from her Durham, N.C., home. “And then they couldn’t reach them by radio.”

The tug’s owner, K-Sea Transportation Partners of East Brunswick, N.J., declined to identify the mate or describe the crew’s experience level. Nor would K-Sea 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.