Russian security forces have arrested four men allegedly linked to the Islamic State (IS) terror organisation and suspected of planning a series of terror attacks on Moscow’s transport system, authorities said on Thursday.

According to a statement by the Federal Security Service (FSB, formerly KGB), four men, including citizens of Russia and of unspecified Central Asian countries, were plotting to use homemade explosive devices to attack Moscow’s public transportation infrastructure on the orders of IS leaders in Syria.

“The terrorists planned to travel to Syria after committing a sabotage and terrorist attack,” the FSB said.

The agency said a bomb-making laboratory and several explosives were found during the raid that led to the four suspects’ arrest, Efe news reported.

It added that extremist literature, automatic firearms, ammunition and hand grenades were also found over the course of the arrest.

On April 3, a suicide bomber detonated an improvised explosive hidden in his briefcase in St. Petersburg’s metro, killing 15 people and injuring at least 45 others.

The Kremlin is worried that terrorists may target this summer’s FIFA Confederations Cup and the 2018 World Cup, both of which will be hosted by Russia.

President Vladimir Putin has ordered an increase in security measures for the upcoming Confederations Cup, which is set to be played in the cities of St. Petersburg, Moscow, Kazan and Sochi between June 17 and July 2.