In this aerial photo, emergency personnel work at the scene of a deadly train wreck Wednesday in Philadelphia. Officials say the train was traveling twice the speed limit.

Some people could remain unaccounted for Wednesday after Tuesday night's train wreck in Philadelphia, though some passengers listed on the manifest might not have boarded the train.

Emergency personnel work the scene of an Amtrak train wreck Tuesday in Philadelphia.

Emergency personnel help a passenger at the scene of a train wreck Tuesday in Philadelphia.

Emergency personnel work to check for victims of the Amtrak train wreck Tuesday night.

Emergency personnel work the scene of a train wreck Tuesday in Philadelphia.

Emergency personnel help victims of an Amtrak train wreck Tuesday night in Philadelphia that injured more than 200 people and killed seven.

Emergency personnel work the scene of the Amtrak train wreck Tuesday night in Philadelphia.

Surrounded by friends and family, Howard and Susan Zemser, the parents of U.S. Naval Academy Midshipman Justin Zemser, prepare to speak to the media outside their home in New York on Wednesday. Zemser, 20, who was on leave and heading home to Rockaway Beach, N.Y., was killed in Tuesday night's Amtrak passenger train derailment.

Emergency personnel remove victims of a train crash Tuesday night in Philadelphia.

Amtrak suspended all service Wednesday along the Philadelphia-to-New York stretch of the nation's busiest rail corridor after Tuesday night's deadly train derailment and crash.

The notoriously tight curve where an Amtrak passenger train crashed Tuesday night is not far from the site of one of the deadliest train wrecks in U.S. history in 1943, which killed 79 people.

Investigators examine the train derailment site Wednesday in the Port Richmond section of Philadelphia. Federal investigators say the train was speeding.

In this aerial photo, emergency personnel work at the scene of a passenger train wreck in Philadelphia on Wednesday.

In addition to the data recorder, the train that crashed in Philadelphia had a video camera in its front end that could yield clues to what happened.

Philadelphia Mayor Michael Nutter, center left at podium, speaks at a news conference near the scene of a deadly train wreck Wednesday in Philadelphia.