Russian Derailment Was Terrorist Attack

The derailment last night of a high-speed Moscow-to-St. Petersburg train that killed more than 25 people apparently was a terrorist attack, Russian authorities say:

Russian officials opened a terrorism investigation Saturday, saying that a homemade bomb planted on the tracks of the high-speed Moscow-to-St. Petersburg route caused a derailment that killed at least 26 people and injured dozens more.
The head of Russia’s Federal Security Service, Alexander Borotnikov, was quoted by the Interfax and RIA Novosti news as saying that an improvised explosive device equivalent to 15 pounds (7 kilograms) of TNT had detonated when the train passed over it Friday night. Remains of the device were found at the site of the crash, Borotnikov said.

Anything said by the Russian government should be taken with a grain of salt, but in this case the claim appears plausible, based on witness statements and what is known of the evidence at the scene:
The most obvious suspects are Chechan Muslims, but so far no claim of responsibility has been reported.