MOSCOW – Two female suicide bombers blew themselves up on Moscow's subway system as it was jam-packed with rush-hour passengers Monday, killing at least 37 people and wounding 102, officials said.

The head of Russia's main security agency said preliminary investigation places the blame on rebels from the restive Caucasus region that includes Chechnya, where separatists have fought Russian forces since the mid-1990s.

The first explosion took place just before 8 a.m. at the Lubyanka station in central Moscow. The station is underneath the building that houses the main offices of the Federal Security Service, or FSB, the KGB's main successor agency.

A second explosion hit the Park Kultury station about 45 minutes later. Emergency Minister Sergei Shoigu said the toll was 37 killed and 102 injured, but he did not give a breakdown of casualties at each station, according to Russian news agencies.