Police and emergency officials had initially attributed the Sunday night explosion at the Moscow Institute of Transport Engineering dormitory to a gas leak.
Moscow police spokesman Yevgeny Gildeyev said, however, that an explosive had detonated on the street outside the entrance to the dormitory basement.
Prosecutors have opened a criminal investigation on charges of ``malicious hooliganism,'' he said. Investigators are considering various motives but the most likely is an attempt to establish control over business, Gildeyev said.
The force of the blast was equal to about 17.5 ounces of TNT, he said.
The explosion shattered most windows in the eight-story building in northeastern Moscow, but there was no fire and the building didn't suffer major structural damage. Five hundred people were evacuated and nine of the injured, including one child, were hospitalized, Gildeyev said.
The building serves as a dormitory for students of the transport institute and houses Vietnamese traders who rent rooms in one of its wings. Foreign vendors often rent out rooms in such dormitories, where prices are low and cash-strapped universities are looking for extra revenue.
Most Russian homes use natural gas for cooking, and leaks causing explosions are common in deteriorating housing stock, killing scores of people each year.
