Russian President Vladimir Putin said on Saturday that the four men who attacked a concert near Moscow were heading toward Ukraine when they were detained, and that they hoped to cross the border.

Putin, addressing the nation, said that some people on the Ukrainian side had prepared to let them cross the border from Russia. Ukraine has denied any involvement in the attack.

“They tried to hide and moved toward Ukraine, where, according to preliminary data, a window was prepared for them from the Ukrainian side to cross the state border,” Putin said.

Putin cast the enemy as “international terrorism” and said that he was ready to work with any state which wanted to defeat it.

“All the perpetrators, organizers and those who ordered this crime will be justly and inevitably punished. Whoever they are, whoever is guiding them,” Putin said. “We will identify and punish everyone who stands behind the terrorists, who prepared this atrocity, this strike against Russia, against our people.”

He said March 24 would be a day of mourning.