Russian investigators were in Tajikistan on Tuesday, questioning the families of four men charged with carrying out a deadly attack on a concert hall near Moscow, three Tajik security sources told Reuters.

The sources, who were not authorized to comment publicly, said Tajik security officials had brought the families to the capital Dushanbe from the towns of Vakhdat and Gissar, and from the Rudaki district.

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Tajik President Emomali Rakhmon was personally overseeing the investigation on the Tajik side, the sources said.

On Monday, making his first public comment on Friday’s attack, Rakhmon called it a “shameful and terrible event” and urged Tajiks to protect their children from harmful influences.

Four men of Tajik origin have been remanded in custody on terrorism charges, on suspicion of carrying out the attack.

Three others, also of Tajik origin, were remanded on suspicion of complicity.

ISIS has said it was responsible for the attack and has released video footage that it says shows the massacre. The group has not identified any of the attackers.

Earlier this month, Rakhmon said his government was alarmed by the activity of radical extremist preachers who were “brainwashing” Tajik youths, making them susceptible to manipulation by foreign groups and intelligence agencies.

The former Soviet republic of 10 million is a close ally of Moscow and hosts a Russian military base; its economy depends heavily on remittances from more than a million Tajik migrant laborers working in Russia.

According to the Russian authorities, at least two of the suspects have confessed to taking part in the attack.

The shootings have strengthened anti-immigrant sentiments in Russia, while videos which appeared to show the suspects being tortured prompted a divided reaction. Russia’s commissioner for human rights has called the use of torture against detainees unacceptable. Russian authorities have said they are investigating.

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