Four teenagers aged from 13 to 20 were found dead in the city of Zheleznogorsk in Central Russia's Kursk region, a spokeswoman for the regional police department said.
"Poisoning was cited as a preliminary cause of death, but it is yet unknown what substance caused it," she said.
The bodies of three victims aged 13, 16 and 20 were found in the apartment of their 16-year-old friend, who also died. The boy lived alone as his mother worked under the rotation system in Russia's north.
"Who knows what happened there. All neighbors spoke well of the boy, he and his mother did not look like a problem family," a neighbor said.
"As far as we know, the children died not so long ago. They were seen alive the day before yesterday," he added.