Law enforcement officers secure an area outside a Moscow district court as Russia observed a national day of mourning after the concert hall massacre. AFP

Russia has charged four men for attacking a Moscow concert hall that killed at least 137 people. The country remanded into pre-trial custody the fourth and final gunman in the deadly Crocus City concert hall attack after three of the gunmen were arrested earlier in the weekend have pled guilty to charges of committing a terrorist attack – the deadliest in Russia in two decades.

Muhammadsobir Fayzov was the last suspect to have been arrested Sunday, state news agency TASS reported Monday. The other three who were taken into custody earlier were Dalrdzhon Mirzoyev, Shamsidin Fariduni, and Saidakrami Rachabalizodu.

Fayzov is accused of filming the attack on the concert hall. He reportedly attended a court hearing Sunday on a stretcher and had a hard time talking. A photo published by TASS showed Fayzov with bruises on his face.

All four suspects, who have been remanded into pre-trial detention through May 22, are from Tajikistan, a former Soviet republic. They were in the country either on expired or temporary visas.

As of Sunday, the death toll from the terror attack has risen to 137. The number is expected to rise further. President Vladimir Putin declared Sunday a national day of mourning. He pledged to punish all involved in the deadly attack, which injured at least 182 others – some of whom are still at the hospital and in serious condition. Terror organization Islamic State has claimed responsibility for the attack.

There were a total of 11 people apprehended in relation to the attack, but the abovementioned were the suspected attackers, as per the state news agency. They allegedly attempted to flee towards the Ukrainian border. Putin said Ukraine prepared a "window" for the perpetrators to leave through the border. Kyiv has denied any role in the attack.

Four suspects stormed into the 6,200-seat concert hall Friday night and opened fire at concertgoers who were waiting for Soviet-era rock band Picnic to perform their hit "Afraid of Nothing."

Over 70 ambulance teams were dispatched to the scene as there were significant challenges to rescue efforts, especially after an explosive device ignited a fire within the hall.

Multiple countries, including the U.S. have expressed solidarity with the victims, survivors, and the loved ones of the people whose lives were lost in the terror attack.