STORY: Russia on Sunday charged four men it says are responsible for gunning down scores of people at a concert outside Moscow days earlier– the deadliest attack inside Russia in two decades.

Russian media previously identified the men as citizens of the ex-Soviet republic of Tajikistan living in Russia.

After unverified and brutal videos of the suspect’s interrogations circulated on social media, courtroom images and video published by Russian media showed the suspects bandaged, swollen and bruised, with one man disoriented and struggling to keep his eyes open.

A Moscow court said the four suspects, charged with acts of terrorism, would be remanded in pre-trial custody until late May– adding that three of the four had pleaded guilty to all charges.

The attack has prompted some Russian lawmakers to discuss whether the death penalty should be re-introduced.

Earlier in the day, Russia lowered flags to half-mast for a day of mourning and people continued to lay flowers in memory of those killed.

By Sunday night the death toll had risen to 137, with over 100 still in the hospital, and some in serious condition.

The Islamic militant group Islamic State has claimed responsibility, and released footage of the attack, the location of which Reuters was able to confirm.

Russian authorities said the gunmen were captured near the Ukrainian border after fleeing the concert venue on Friday.

And Putin, who has not yet publicly mentioned Islamic State in connection with the attack, said that some on “the Ukrainian side” had been prepared to spirit the gunmen across the border.

Ukraine has denied any role in the attack.

The U.S. has also denied Ukrainian involvement, saying that Islamic State bore sole responsibility.

The White House said the U.S. government shared information with Russia earlier this month about a planned attack in Moscow, and had issued a public advisory to Americans in Russia.

Russian officials have bristled at the U.S. comments on the attack, and say Russian investigators must be allowed to make their own findings.