Many of the questions to which the NTSB will seek answers revolve around the operator of the incoming train, Jeanice McMillan, 42, of Springfield, Va.
McMillan was hired as a bus driver in 2007 and had only three months' experience as a train operator.
Hersman said the board would look into McMillan's health, whether she was fatigued, and whether she might have been using a cell phone or text-messaging device.
Carol Carmody, a former NTSB member, said such questions are routine in such cases.
"They do blood tests to see if there's any evidence of anything," she said. "You have to wonder what she was doing. Was a cell phone involved?"
Hersman said there was evidence that the operator depressed a "mushroom" button that activates the emergency brake and that the wheels showed wear that indicated an attempt to make a sudden stop.
Fenty said Tuesday that officials had identified all of the passengers who died in the collision. They included one Maryland woman, Ana Fernandez, 40, of Hyattsville; and seven District of Columbia residents, including retired Maj. Gen. David F. Wherley Jr., who headed the D.C. National Guard, and his wife, Ann Wherley, both 62.
The toll in Monday's collision brought the number of people killed in train accidents in the years since the Washington Metro opened to at least 17.
By contrast, Baltimore's much smaller Metro subway has had only one train fatality and no collisions in nearly 26 years of operation, according to Maryland Transit Administration spokeswoman Jawauna Greene. MTA officials said Tuesday that Baltimore's system does not use any of the 1000 Series trains operated by WMATA, which still has about 300 on its tracks. All of its 100 cars, built in the 1980s and overhauled in 2003, have data recorders, according to Jason Lurz, deputy director of Baltimore Metro operations.
Lurz said the Baltimore Metro has four levels of controls to make sure trains runs safely - the human operator, an automatic train operations system, an automatic train protection system and an operations control center. While Lurz expressed confidence in the Baltimore system, he said the MTA has launched an "integrity test" of its systems in response to the Washington collision "just to be proactive."
Metro general manager John B. Catoe Jr. told the board Tuesday afternoon that "this accident should not have happened, and we all know that." He added that two years ago WMATA began a program to improve systemwide safety.
