A homemade bomb sent a high-speed Moscow-to-St. Petersburg train careering off its tracks, killing at least 26 people in what Russian officials said yesterday was an act of terrorism.

Authorities found pieces of an explosive device that detonated Friday night as the train raced over it, gouging a five-foot crater that derailed three carriages of the train as it reached 130 mph.

As many as 100 people were left injured, officials said, and 18 remained missing yesterday.