Russian investigators said on Thursday that they had found evidence that the militants who killed more than 140 people at a concert last week were linked to "Ukrainian nationalists," a claim the United States immediately denied and described as baseless propaganda.

Russia had said from the beginning that it believed Ukraine was connected to the attack, although Kyiv denied this and said the Islamic State militant group was responsible.

The Russian Investigative Committee said in a statement for the first time that it had uncovered evidence of a Ukrainian connection. While it described the nature of the alleged evidence, it did not publish it.

It added that "as a result of working with detained terrorists, studying seized technical devices, and analyzing information related to financial transactions, evidence was obtained linking them to Ukrainian nationalists."

It said there were "confirmed data" that the attackers received large amounts of cash and cryptocurrencies from Ukraine. The committee said another suspect involved in a "terrorist financing scheme" was arrested.

Minutes later, White House National Security spokesperson John Kirby described the Russian accusations as "nonsense and propaganda" and said that the Islamic State was solely responsible for the attack.

The United States had publicly warned earlier in March that it had intelligence indicating "extremists" planning an imminent attack in Moscow. Kirby said they also gave a written warning to Russian security services.

He said, "In fact, the United States tried to help prevent this terrorist attack and the Kremlin knows that."

U.S. officials explained that they believe Islamic State Khorasan, the Afghan branch of the network, is responsible for the concert shooting, while Russia sees the fact that the United States was able to identify the alleged perpetrator so quickly as suspicious.

The head of the Russian Federal Security Service said earlier this week, again without providing evidence, that he believes Ukraine, along with the United States and Britain, are involved.

Western security analysts say the attack raised questions about the resources and priorities of Russian intelligence agencies, which have been heavily focused on the war in Ukraine and the need to eliminate opposition within Russia.

Eleven people were arrested in the first 24 hours after the shooting last Friday, and eight of them, including the four suspected militants, were placed in pretrial detention. Seven of them are from Tajikistan in Central Asia, and the other from Kyrgyzstan.