More than 18 people were killed in an explosion on Monday and three more in the bus explosion the next day under mysterious circumstances in the central Russian city of Magnitogorsk.

Local residents report an increase in security forces activity after deadly Russia explosion in Magnitogorsk and that Police and special forces are looking for terror cells believed to have been involved in Monday’s bomb attack in.

Local residents posted reports about Russian security forces hunt for terror cells and have released some footage including both audio and video files of a bus exploded in the center of Magnitogorsk in the evening of January 1.

Znak.com and “74.ru” with reference to unnamed sources reported that possible traces of explosives were found at the site and that the blast that caused the collapse could have been a terrorist act. And bus explosion is a special operation to kill suspected terrorists.

Video of the apparent attack circulated on social media, but officials declined to comment.

Witnesses also have reported about gunfire, which is clearly audible on the video.

According to official information, three people died as a result of the explosion. According to the Tvrain.ru, one of them lived in the collapsed entrance, traces of hexogen were found in his apartment.

But in a statement issued on January 1, the federal Investigative Committee said the authorities were looking into “all possible causes” and added that no signs of a bomb blast have been found.

“In connection with various reports that have appeared in the media, it must be noted that at the present moment…no traces of explosives or their components have been found,” the statement said.

The residential building in Magnitogorsk collapsed on December 31. According to official information, this happened as a result of the explosion of domestic gas. The Russian government stressed that it did not consider other versions of the incident.

Officials in the city, in the Urals region, say a gas leak caused the blast.

The building was home to 120 people, and 48 flats collapsed in the blast.

Victims and witnesses reported widespread attempts of the Russian government to cover up the terrorist act.