Investigators of Russia's Investigative Committee work in a college in Kerch, Crimea on October 18, 2018, after a student opened fire at a technical college in the Russian-annexed Crimea city of Kerch.
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.
The death toll in Kerch climbed by one Thursday to 20 after one of the wounded died in hospital.
Authorities said they are investigating whether Roslyakov had accomplices or whether he was acting "under the influence" of another person or group.
The Kerch attack was also the deadliest school violence in Russian Federation since the 2004 Beslan attack by Chechen separatists, which left 333 people dead, many of them children, during a three-day siege.
Numerous victims from Wednesday's attacks were teenage students who suffered shrapnel and bullet wounds.
Since then, fighting in that region has killed more than 13,000 people, including about 3,000 civilians. Russia's National Guard said Thursday it has now deployed officers and riot police to all schools and colleges in Kerch.
However, the Investigative Committee, the state body that investigates major crimes, said later that it was re-classifying the case from terrorism to mass murder.
On Wednesday, members of the Russian parliament stood for a minute of silence during their session, and Russian President Vladimir Putin gave his condolences to the relatives of the victims of the "tragic event".
The attack had first been reported as an explosion, with Russia's National Anti-Terrorism Committee saying an "unidentified explosive device" had gone off.
Roslyakov's body was discovered in the college library, according to authorities.
In other media, the blame for the Kerch attack was more directly focused.
Guns are tightly restricted in Russian Federation. Roslyakov received a permission to own a shotgun and bought 150 cartridges just a few days ago, according to local officials.
In September, Vladislav Roslyakov (18) got his authorization to possess weapons.
"A shooting took place after the explosion". He said he looked out and saw a young man with a rifle shooting at people.
"I locked the door, hoping he wouldn't hear me", the paper quoted Gavrilov as saying.
No motive has officially been given for the attack, which recalled similar shooting sprees carried out by students in United States schools.
In April, another student stabbed a teacher and a fellow student in the Urals Mountains and then set fire to a classroom.
"The suspected attacker shot himself".
In another attack in January, two teenagers stabbed children and their teacher with knives, wounding 15 people, and then attempted to kill each other before being detained.
Bomb disposal experts of the FSB have been deployed to the college in Kerch, Crimea, a Black Sea peninsula which Russian Federation annexed from Ukraine four years ago.
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