Four Daesh terrorists, who were plotting attacks on public transportation were arrested Thursday in Moscow, Russia's Federal Security Service (FSB) said.

The group consisted of four people, including Russian nationals and citizens of Central Asian countries.

During a search, the security services found a hand-made bomb, materials for making explosive devices, as well as firearms, ammunition, grenades, literature and video recordings of "terrorist orientation."

The FSB said that the suspects were instructed by someone from Syria.

"After staging a terror attack, the terrorists planned to go to Syria to fight as part of the international terrorist organization Daesh," the security service said.

15 people were killed by a suicide bomber in an attack on the St. Petersburg metro in April.