Russia’s Federal Security Service (FSB) said on Monday it had detained members of a terrorist cell who were planning attacks on Moscow’s public transport system.

The statement comes nearly two months after the FSB said it had detained four people suspected of organizing attacks in Moscow and shut down a bomb-making lab on the outskirts of the city.

The FSB said in late August it had foiled terrorist attacks on the first day of school, Sept. 1.

The FSB said Monday’s detainees were part of an Islamic State sleeper cell led by “foreign emissaries." The Islamic State is a terrorist group banned in Russia.

Two homemade explosive devices were seized alongside two handguns, two grenades and other explosive devices at the suspects’ residence in the Moscow region.

The detainees are Russian citizens from the North Caucasus, the FSB said. They are alleged “supporters of the forced unification of all Muslims to create a so-called ‘Worldwide Islamic Caliphate,'” they added.

The FSB did not specify the number of suspects detained, but footage published by Russian media depicted at least three individuals being interrogated.

The latest detentions are part of a series of raids in the wake of an April 3 terrorist attack on the St. Petersburg metro that killed 15 people and wounded dozens.