Moscow, 23 March

Russian authorities arrested the four people suspected of taking part in the attack on a suburban Moscow concert hall that killed at least 133 people and believe they were headed to Ukraine, President Vladimir Putin said Saturday during an address to the nation.

Kyiv, meanwhile, strongly denied any involvement in Friday's attack on the Crocus City Hall music venue in Krasnogorsk, which the Islamic State group's affiliate in Afghanistan claimed responsibility for in a statement posted on social media channels linked to the group. Kyiv accused Putin and other Russian politicians of falsely linking Ukraine to the assault in order to stoke fervor in Russia's war in Ukraine, which recently entered its third year.

A US intelligence official told The Associated Press that US agencies had confirmed that IS was responsible for the attack. Putin said authorities have detained a total of 11 people in the attack, which also injured scores of concertgoers and left the venue a smoldering ruin. He called it “a bloody, barbaric terrorist act” and said Russian authorities captured the four suspected gunmen as they were trying to escape to Ukraine through a “window” prepared for them on the Ukrainian side of the border.

Putin also said that additional security measures have been imposed throughout Russia, and he declared Sunday to be a day of mourning.

Investigators on Saturday were combing through the charred wreckage of the hall for more victims, and the authorities said the death toll could still rise. Hundreds of people stood in line in Moscow early Saturday to donate blood and plasma, Russia's health ministry said.

“We faced not just a thoroughly and cynically prepared terror attack, but a well-prepared and organized mass murder of peaceful innocent people,” Putin said.