Russian investigators said on Thursday they had found proof gunmen who killed more than 140 people at a concert last week were linked to "Ukrainian nationalists", an assertion immediately dismissed by the US as baseless propaganda.
Moscow has said from the outset it believes Ukraine was linked to the attack, even though Kyiv has denied it and the militant group Islamic State has claimed responsibility.
In a statement, Russia's Investigative Committee said for the first time it had uncovered proof of a Ukrainian connection. 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 analysing information about financial transactions, evidence was obtained of their connection with Ukrainian nationalists," the committee said.
It said there was "confirmed data" the attackers had received significant amounts of cash and cryptocurrency from Ukraine. Another suspect involved in "the terrorists' financing scheme" had been detained, the committee said.
White House national security spokesperson John Kirby described the Russian allegations as "nonsense and propaganda" and said Islamic State was solely responsible for the attack.
"In fact, the United States tried to help prevent this terrorist attack and the Kremlin knows this."
The US had publicly warned earlier in March it had intelligence "extremists" were planning an imminent attack in Moscow. Kirby said it had also provided written warning to Russian security services on March 7 at 11:15am.
Revealing the exact time of the alert appeared aimed at neutralising Russia's charges against Ukraine. Three days before the attack, Putin dismissed Western warnings of a potential extremist attack as "blackmail".
US officials have said they believe it was Islamic State Khorasan, the network's Afghan branch, that was responsible for the concert shooting.
"We take seriously our duty to warn. We never want to see innocent lives lost in terrorist attacks," said Kirby, adding Washington has passed "many warnings" to Moscow "about various threats" since September.
Russia has said the fact the US was able to name the alleged perpetrator so fast is suspicious.
The head of Russia's FSB security service said earlier this week, again without providing evidence, he believed Ukraine, along with the US and UK, were involved.
Western security analysts have said 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.
Eleven people were arrested in the first 24 hours after last Friday's shooting 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 from Kyrgyzstan.
