An idled tour boat and nearby vessels made repeated calls to the tugboat guiding the massive barge that hit and sunk the craft, killing two Hungarian students, according to a preliminary report issued today by the National Transportation Safety Board.

The NTSB said it found the stricken amphibious tour boat's radio calls to the approaching tug went unanswered in the moments before the collision in the Delaware River on July 7.

There were 35 passengers and two crew members aboard the Ride the Ducks amphibious boat when it was hit just south of the Benjamin Franklin Bridge.

The barge plowed over the duck boat, sinking it and sending passengers into the water. Two Hungarians drowned.

According to the report, drug and alcohol tests on the crews of both vessels were negative.

The mate piloting the tugboat declined to answer investigators' questions, citing his Fifth Amendment right against self-incrimination.