Experts suspected speed was to blame.
''If the train hadn't hit anything before derailing ... the train was probably speeding. For the train to flip, it had to be traveling at a high speed,'' Kazuhiko Nagase, a Kanazawa Institute of Technology professor and train expert, told NHK.
NHK reported that the automatic braking system at that stretch of track is among the oldest in Japan. The system stops trains at signs of trouble without requiring drivers to take emergency action, but the older system can't halt trains traveling at high speeds, NHK said.
Tokyo dispatched Self-Defense Force soldiers to the disaster scene to assist. Prime Minister Junichiro Koizumi offered condolences to families of passengers who were killed, as did Emperor Akihito, in unusual unscripted remarks. Koizumi pledged that officials would do everything they could to prevent a recurrence of the crash.
''It's tragic,'' Transport Minister Kazuo Kitagawa said at the scene. ''We have to investigate why this horrible accident happened.''
Deadly train accidents are rare in Japan. Five people were killed and 33 were injured in March 2000, when a Tokyo subway hit a derailed train. An accident killed 42 people in April 1991 in Shigaraki, western Japan.
An earthquake in 2004 caused a bullet train to derail, the first since the high speed trains went into service 40 years ago.