Failed brakes may have caused an Amtrak train to derail and smash into a commuter train, injuring 264 people, federal safety officials said.

They also said Thursday that the Amtrak train may have been traveling too fast along a curve where the crash occurred, but that the speed may never be determined because one speed recorder on the train was not working and the other was destroyed in the crash.An apprentice engineer operating the Amtrak train told investigators he tried to brake three times as the Night Owl from Washington approached a Boston station, said Susan Coughlin, vice chairwoman of the National Transportation Safety Board.

"He indicated that in his three attempts to brake the train, the brakes failed to set up properly to stop it," Coughlin said at a news conference.

The apprentice, Richard Abramson, also told authorities that senior engineer Willis Copeland tried to stop the Night Owl using emergency brakes, she said.

Coughlin said brakes on Amtrak cars that did not derail were working normally. Brakes from the derailed cars will be tested.

The rush-hour crash Wednesday morning near the Back Bay Station injured 264 people. Copeland, 53, of Bethany, Conn., and Abramson, 41, of Trumbull, Conn., were among 11 of those Thursday still hospitalized.

Coughlin said that one of the Night Owl's speed recorders was destroyed in the crash and the other, for unknown reasons, did not record during the last five miles.

"We will never know the exact speed of impact. I regret the fact that we don't have" that data, she said. "The last time he (Abramson) looked at the speedometer, he was going between 60 and 70 mph," she said.

Coughlin said data from signals tripped by the train showed it traveled the last 11/4 miles in 49 seconds at an average speed of 91.8 mph. Before that last stretch, the train could travel safely at 100 mph. But it should have slowed down to 30 mph by the time it reached the point where it derailed, on a curve 500 feet from the station, she said.