The Duck boat experienced mechanical difficulties and was anchored in the Delaware River about 2:35 p.m. Wednesday when the 75-foot tugboat, the Caribbean Sea, pushed a 250-foot sludge barge, The Resource, over the Duck boat.
All but two of the 37 people on board the Duck survived as the vessel sank 55 feet. The bodies of Hungarian students Dora Schwendtner, 16, and Szabolcs Prem, 20, were recovered from the river Friday.
Crew members on the Duck boat and operators of other boats in the water at the time told NTSB investigators that the Duck boat got no response when it tried to call the Caribbean Sea on a ship-to-ship channel.
Of the five crew members on the tugboat - a master, the first mate, an engineer and two deckhands - only three interviewed with investigators, the NTSB said. Aside from the first mate, a deckhand was not interviewed because he said he was sleeping at the time of the crash, the NTSB said.
NTSB spokesman Keith Holloway said it was not unusual for a deckhand to be sleeping.
"In this case, we didn't find anything abnormal about it," he said. "They have an on-and-off type of service they follow, so it wasn't abnormal for someone to be asleep at that particular time of the day."
Doug Dillon, executive director of the Tri-State Maritime Safety Association, agreed.
"Typically, you only have to have two people - the person in charge of navigating and the engineer," he said. "I certainly wouldn't read into the fact that someone was sleeping."
Dillon said it was unusual that a crew member would invoke his Fifth Amendment rights in an NTSB investigation.
"A lot of the NTSB's accidents are mechanical in nature. With their investigations, mostly you don't see a lot of refusal," he said. "In this particular case, based on some of the questions that have come out, it's not a mechanical issue they're looking at - it's a personnel issue."
Holloway said the NTSB was also examining still and video footage taken from tourists and surveillance cameras and was analyzing the tugboat's GPS and electronic chart navigation systems.
Amid the investigation and unanswered questions, the 11 surviving Hungarian students and their two teachers headed home with their own unanswered questions yesterday morning. John Oostdyk, director of Atlantic Bridge, which arranged the tour, shared some of the questions on the group's Web site yesterday.
"Can we begin to look at this all from a different angle? Is there a deeper meaning behind the events of the past week?" he wrote. "How are we to understand it?"