Russia has arrested more than 20 people in connection with an attack claimed by Islamic State that killed more than 140 people at a concert hall near Moscow in March, the head of the FSB security service said on Friday.

Alexander Bortnikov repeated Russia’s assertion, for which it has not provided evidence, that Ukraine was behind the attack. Kyiv has dismissed the allegation as absurd.

“The investigation is ongoing, but it is already safe to say that Ukrainian military intelligence is directly involved in this attack,” state news agency TASS quoted Bortnikov as saying.

Four attackers gunned down concertgoers at the Crocus City Hall near Moscow on March 22 and then set fire to the venue, in the deadliest attack in Russia for 20 years.

While pointing the finger at Ukraine, Bortnikov also said, according to Russian news agencies, that the preparation, financing, the actual attack and the attempted flight of the gunmen were coordinated over the internet by Islamic State Khorasan, the Afghan branch of the militant group.

Andriy Yusov, spokesperson for Ukraine’s military intelligence agency, said on national television on Thursday:

“Russian propaganda did not forget about the Crocus story. It will continue trying to promote the absurd point about Ukraine’s alleged complicity even though it can’t stand up to any criticism and it’s completely illogical.”

The suspects held by Russia include the four suspected gunmen, all citizens of Tajikistan, whose detention was extended last week until Aug. 22. No date has been set for their trial.