A Mexican naval ship in the East River accelerated suddenly in the wrong direction before slamming its masts into the Brooklyn Bridge in a crash that killed two crew members, federal transportation officials said on Monday.

The ship, the Cuauhtémoc, was moving at a speed of about 2.3 knots after shoving off from a Lower Manhattan pier Saturday night with a tugboat’s help, Brian Young of the National Transportation Safety Board said at a news conference.

The 300-foot long ship, which had 277 people on board, maintained that pace for “a bit of time” before “the speed began to increase,” said Mr. Young, the investigator leading the safety board’s inquiry into the crash. The Cuauhtémoc’s speed had risen to six knots when it hit the bridge less than five minutes after leaving shore, he said.

It was unclear what caused the sudden acceleration, Mr. Young said. But it will be among the issues investigators focus on in the course of an examination that is in its earliest stages and that could take up to two years to complete.

“This is a start of a long process,” Michael Graham, an N.T.S.B. board member, said at the news conference, noting that the agency expected to issue a preliminary report of its findings within 30 days. “We will not be drawing any conclusions. We will not speculate.”

Mr. Graham said agency officials were working with their Mexican counterparts to gain access to the ship so that investigators could inspect the engine, interview crew members still on board and recover any data recorders the vessel may have. The damaged vessel is now docked at Pier 36 in Manhattan.