Russian investigators said on Thursday they had uncovered evidence that the gunmen who killed more than 140 people in an attack on a concert hall near Moscow last week were linked to "Ukrainian nationalists."
Russia has said from the outset that it is pursuing a Ukrainian link to the attack, even though Kyiv has denied it and the militant group Islamic State has claimed responsibility.
The state Investigative Committee said in a statement for the first time that it had uncovered evidence of a Ukrainian link. While it described the nature of the alleged evidence, it did not publish it.
"As a result of working with detained terrorists, studying the technical devices seized from them, and analyzing information about financial transactions, evidence was obtained of their connection with Ukrainian nationalists," the statement said.
It said the attackers had received significant amounts of cash and cryptocurrency from Ukraine and that another suspect involved in terrorist financing had been detained.
Eleven people were arrested in the first 24 hours after last Friday's attack, and eight of these, including the four suspected gunmen, have been placed in pre-trial detention. Seven are from the Central Asian state of Tajikistan, and the other is from Kyrgyzstan.
The United States had publicly warned before the concert shooting that it had received intelligence of a possible attack by extremists in Russia. US officials say they believe it was Islamic State Khorasan, the network's Afghan branch, that was responsible.
Russia says it is suspicious that the US was able to name the alleged perpetrator of the attack so soon after it took place. The head of Russia's FSB security service said earlier this week, again without providing evidence, that he believed Ukraine, along with the US and Britain, were involved.
Western security analysts say the attack raised questions about the resourcing and priorities of Russian intelligence agencies that have been heavily focused on the Ukraine war and the need to stamp out opposition to it within Russia.
US response to Russia's claims
The White House on Thursday dismissed as "nonsense" Russia's charge of Ukraine's involvement in last week's attack on Moscow's Crocus City concert hall that claimed more than 140 lives, saying it was clear that Islamic State was "solely responsible."
In a briefing to reporters, White House national security spokesman John Kirby said that the United States passed to Russian security services a written warning of an extremist attack on large gatherings in Moscow, one of many provided in advance.
"It is abundantly clear that ISIS (Islamic State) was solely responsible for the horrific attack in Moscow last week," Kirby said. "In fact, the United States tried to help prevent this terrorist attack, and the Kremlin knows this."
Kirby spoke shortly after Russia's Investigative Committee said it had uncovered evidence that the four gunmen who carried out last Friday's attack were linked to "Ukrainian nationalists" and had received cash and cryptocurrency from Ukraine. He described the Russian allegations as "nonsense and propaganda."
The United States, he said, provided multiple advance warnings to Russian authorities of extremist attacks on concerts and large gatherings in Moscow, including in writing on March 7 at 11:15 am, to Russia's security services.
The United States "following normal procedures and through established channels that have been employed many times previously... [passed] ... a warning in writing to Russian security services," Kirby said.