An explosive device rigged with metal fragments has killed 18 people at a college in Crimea, said Russia’s top investigative agency on Wednesday.

The Russian Investigative Committee’s spokeswoman, Svetlana Petrenko, said the device went off on Wednesday in the canteen of a vocational college in the Black Sea city of Kerch. She said most of the victims were students and more than 40 people were injured as a result of the explosion.

Russian news media, however, reported that at least some of the victims died in an attack by an unidentified gunman or gunmen. Russian officials wouldn’t confirm those reports.

Dmitry Peskov, the spokesman for Russian President Vladimir Putin, told reporters that officials are looking into a possible terrorist attack. He did not elaborate.

The National Anti-Terrorism Committee said in a statement that the blast at the college was caused by an explosive device.

The committee's spokesman Andrei Przhezdomsky said sappers are inspecting the building for other explosive devices.

The Tass news agency reported that most of the victims were students, quoting local ombudswoman Lyudmila Lubina.

Emergency officials had earlier said that the blast was caused by a gas canister explosion.

The head of Crimea, Sergei Aksyonov, and Russia's Health Minister Veronika Skvortsova headed to the area to coordinate assistance to those injured.

Russia has annexed Crimea from Ukraine in 2014, a move that triggered Western sanctions.