A shrapnel-filled bomb exploded in a McDonald's restaurant in St Petersburg on Sunday night, injuring six people and wrecking the popular downtown eatery.
Police say the bomb, which was carried in a woman's handbag, went off with the force of 75 grams of TNT, shattering windows and bringing part of the ceiling crashing down.
"A preliminary expert examination revealed that the homemade explosive device was stuffed with preformed fragments," a police source told the official RIA-Novosti agency.
Six people were hospitalised, including a German tourist, with injuries ranging from lacerations to concussion.
Investigators downplayed initial reports that the blast may have been an act of terrorism, suggesting that a criminal feud may have been the cause. A criminal investigation into "attempted murder" has been launched, according to the independent Interfax agency.
McDonald's restaurants have been frequent targets for both terrorist bombers and criminal vendettas in Russia.
A car bombing at a Moscow McDonald's in 2002, attributed to Chechen terrorists, killed one and injured eight. Another St Petersburg McDonald's was hit by a powerful blast while it was under construction in 1998, but no one was in the building at the time.