Russia on Friday said for the first time that the militant Islamic State group coordinated the March concert hall assault in Moscow, the country’s deadliest terror attack in two decades.

IS has claimed responsibility on multiple occasions for the March 22 attack which killed over 140 people, but Moscow has repeatedly tried to link Ukraine and the West to the attack.

FSB chief Alexander Bortnikov was quoted as saying by the RIA Novosti news agency that “preparations, the financing, the attack and the retreat of the terrorists were coordinated via the internet by members of Khorasan Province (IS-K)”, an IS branch active in Afghanistan and Pakistan.

Bortnikov did not discard the Ukrainian angle in his statements on Friday, saying that “upon completing the attack, the terrorists received clear instructions to move toward the Ukrainian border, where from the other side a ‘window’ had been prepared for them”.

“The investigation continues, but it can already be said with certainty that Ukrainian military intelligence is directly implicated in the attack”, he said.

Ukraine has repeatedly denied involvement.

Gunmen in camouflage stormed the Crocus City Hall venue on the outskirts of Moscow before setting the building on fire.

More than a dozen suspects have been arrested including the four assailants, who are all from the Central Asian nation of Tajikistan, an impoverished former Soviet republic on Afghanistan’s northern border.

The United States has said it had publicly and privately warned Russia in early March that extremists were planning an attack on a concert hall in Moscow.

Unnamed US intelligence officials told American media outlets after the massacre that they had told Moscow it was the Crocus City Hall specifically that IS was planning to attack.

Russia dismissed those warnings. Just three days before the attack, President Vladimir Putin accused Washington of “blackmail” and trying to “intimidate” Russians.