MOSCOW, April 23. /TASS/. There is no denying Ukraine's connection to the Crocus City Hall terrorist act in Moscow, because someone promised the perpetrators of the deadly attack an open "window" at the Ukrainian border, Former Ukrainian Security Service (SBU) officer Vasily Prozorov told TASS on Tuesday.

He added, however, that the perpetrators could have been eliminated once they crossed the border.

"There is a Ukrainian connection, without a doubt," he said. "As far as I know, there was a ‘window’ open along the terrorists' escape route on the Ukrainian border and there was a group waiting for them there."

"One can only guess what this meeting would have looked like," Prozorov continued. "Perhaps, they could have been greeted, rewarded and transported as promised."

"Perhaps, the [Ukrainian] special services were simply tasked to eliminate them and proudly announce later that ‘we are also fighting against terrorism, we are good.’ I lean towards the second scenario," he added.

On the evening of March 22, a terrorist attack targeted the Crocus City Hall music venue in Krasnogorsk, Moscow Region, just outside the Moscow city limits. According to the latest data, 145 people were killed and 551 people were injured.

Eleven people suspected of involvement in the terrorist attack have been apprehended, including four direct perpetrators. The Russian Investigative Committee announced that it had proved that the perpetrators of the attack were linked to Ukrainian nationalists.