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 (Daesh) has claimed responsibility.

In a statement, the state Investigative Committee said for the first time that it had uncovered evidence of a Ukrainian link, Reuters reported.

“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.

However, the White House has described Russia’s allegation that Ukraine was involved in the attack as “nonsense”.

National security spokesman John Kirby said it was clear that ISIS was “solely responsible.”

He added the US passed a written warning of an extremist attack to Russian security services, one of many provided in advance to Moscow.

Russia says however it is suspicious that Washington 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 U.S. and Britain, were involved.