But President Vladimir V. Putin of Russia held fast to the idea that a Western-backed Ukraine could have been the ultimate mastermind of the assault that killed 139 people.
President Vladimir V. Putin on Monday acknowledged for the first time that the bloody assault on a concert hall near Moscow was executed by “radical Islamists,” but he insisted that Ukraine could still have played a role despite the Islamic State’s claim of responsibility.
As Russians grieved, bringing flowers and candles to makeshift memorials across the country, Mr. Putin said that the tragedy, the worst such attack in the capital in two decades, was likely ordered by Ukraine.
“The question is: Who benefited from it?” Mr. Putin said on Monday evening during a publicly broadcast meeting with government officials. “This atrocity can be just an element in a series of attempts of those who have been at war with our country since 2014,” he said, referring to the Ukrainian government.
Russian investigators have not disclosed any evidence demonstrating that the four suspects, men from Tajikistan who were migrant workers in Russia, have a connection to Ukraine.
In the news reports, Russian state television presented the location of their arrest — the Bryansk region of Russia that borders Ukraine — as evidence of Ukrainian involvement. The reports also suggested that Kyiv could have hired them to mount the attack.