Friends silently and solemnly dropped white roses into the Delaware River on Saturday in memory of the two Hungarian students who died when the amphibious tourist boat they were riding was struck by a barge and sank.

City officials, religious leaders and Hungarian diplomats joined a group of grieving Hungarian exchange students at a memorial service dedicated to 20-year-old Szabolcs Prem and 16-year-old Dora Schwendtner, who drowned after the duck boat capsized Wednesday.

"The loss of a young life, of two young lives, is almost impossible to understand and almost impossible to accept," said Bela Szombati, the Hungarian ambassador to the United States. "We stand with you, we stand with the children, the young people."

At the end of the ceremony, wreaths and flowers were dropped into the river and a pair of doves were released.

Both bodies were recovered from the river Friday and identified based on their personal effects, said Jeff Moran, spokesman for the Philadelphia medical examiner's office.

The two crew members and 33 other passengers on the duck boat were rescued from the river. Eleven of the survivors were Hungarian students. Two Hungarian teachers and seven Americans who were touring with them also survived.

The National Transportation Safety Board sent a team of 10 investigators to Philadelphia to piece together how the crash occurred. They say the duck boat's captain shut off the engine and dropped its anchor after smoke billowed from the vessel. The boat was stopped in the water for five to 10 minutes before the barge, being pushed by a tugboat, struck it. It capsized within seconds.

Among the issues to sort out: whether the tugboat crew heard distress calls that the duck boat crew says it made, and why an air horn on the smaller vessel apparently failed.

The NTSB, which said it would have no more briefings on its investigation, was conducting interviews Saturday with the five crew members of the tugboat.