Reports are mounting that Vladimir Putin’s government knew of impending carnage in Moscow, but did not react. Kremlin security services knew of an ISIS threat days before the attack on a concert hall near Moscow, according to Russian intelligence documents cited by a UK-based group.

According to the London-based Dossier Center, the documents indicated that ethnic Tajiks of ISIS-K (Islamic State of Khorasan, an offshoot of ISIS) could be involved.

At least 143 people were killed last Friday in the deadliest attack in Russia in decades, when attackers stormed Crocus City Hall with guns and incendiary devices, shortly before a concert. ISIS claimed responsibility for the attack with statements, photos and a propaganda video filmed by the attackers.

The Dossier Center is a Russian research group supported by the Mikhail Khodorkovsky, an exiled Russian former oil tycoon critical of the Kremlin. He has previously exposed details about Russian President Vladimir Putin and his regime, often using documents and leaks from within the Russian government.

“A few days before the terrorist attack, members of the (Russian) Security Council received a warning that citizens of Tajikistan could used in terrorist attacks on Russian territory,” the group’s latest report, released Sunday, said, citing the Russian security service. “Even before the Crocus attack, a source close to the intelligence services told the Dossier Center about it,” he added.

The Kremlin has not responded to CNN’s request for comment on the Dossier Center report. Despite the fact that relations between Washington and Moscow are at historic lows, the United States announced that they warned Russia that ISIS fighters were planning to attack the country. Earlier in March, the US Embassy warned of an increased threat of terrorist attacks in Russia, with National Security Council spokeswoman Adrienne Watson saying the US had shared that information with Russian authorities as part of its “obligation to warn” policy. But in a speech days before the attack, Putin had dismissed the US warnings as “provocative”, saying “these actions look like direct blackmail and the intention to intimidate and destabilize our society”. Putin, who recently won an election to secure another term, has repeatedly suggested, without evidence, that Ukraine helped orchestrate the attack. Ukraine has repeatedly refused that it has anything to do with the attack, saying Moscow had been informed of an imminent terrorist strike.