Russia has said for the first time that the Daesh terrorist group coordinated the concert hall assault in Moscow in March, the country's deadliest terror attack in two decades.
Several gunmen burst into the big concert hall and fired automatic weapons at the crowd which left more than 140 people dead.
"The investigation is ongoing, but it is already safe to say that Ukrainian military intelligence is directly involved in this attack," head of Russia's Federal Security Service (FSB) Alexander Bortnikov said Friday.
The FSB chief pointed out that NATO forces and Ukraine military intelligence had used Daesh militants to fight against Russian troops and target interests of the country.
NATO, Bortnikov said, facilitated the transfer of "mercenaries and militants of international terrorist organizations from the Middle East, North Africa and Afghanistan" to Ukraine.
On Thursday, the FSB said it had arrested three more suspects for complicity in last March' terrorist attack on the concert hall.
"Two of those detained transferred money for the purchase of firearms and vehicles used in the terror attack, and a third was directly involved in recruiting accomplices of the terror attack and financing its perpetrators," the Interfax news agency quoted the FSB as saying in a statement.
Russian state media outlets showed footage of the FSB agents who were making the arrests. Of those arrested, two were foreign citizens and one was from Russia, the FSB revealed.
Till now, Russian security forces have arrested more than a dozen people involved in the attack, including the four gunmen, all citizens of Tajikistan.
Daesh has repeatedly claimed responsibility for the attack, but the Russian government insists that it had been "ordered" by either Kiev or the West.
Russian President Vladimir Putin on Thursday said the raison d'etre of extremist groups like Daesh was contrary to "the bloody, horrific terror attack in Moscow".
The Russian leader said those who orchestrated the Moscow attack aimed "to inflict damage on our unity."