MOSCOW 

At least 11 people, including four involved in Friday's attack at a concert hall in Moscow region have been arrested, head of Russia's Federal Security Service (FSB) Aleksandr Bortnikov has told President Vladimir Putin, according to the Kremlin.

The FSB is working on identifying the accomplice base of the perpetrators of the terrorist attack in Crocus City Hall, the Kremlin press service quoted Bortnikov as saying on Saturday.

Gunmen opened fire at concertgoers late Friday, killing at least 93 people and injuring 145 others. The attack took place as rock band Picnic was about to perform at the Crocus City Hall just west of Moscow.​​​​​​​