Moscow, Russia (CNN) -- Female suicide bombers detonated explosions that rocked two subway stations in central Moscow during rush hour on Monday morning, killing at least 35 people, officials said.
"The blast was caused by 300 to 400 grams of explosives," he said.
A Web site associated with Chechen separatists claimed responsibility for the attacks. Chechnya is located in an area of Russia known as the North Caucasus, located between the Black and Caspian Seas.
Forensic teams were combing wreckage from the underground blast for clues.
"Our preliminary assessment is that this act of terror was committed by a terrorist group from the North Caucuses region," said Alexander Bortnikov of the Federal Security Service. "We consider this the most likely scenario, based on investigations conducted at the site of the blast.
"Fragments of the suicide bombers body found at the blast, according to preliminary findings, indicate that the bombers were from the North Caucuses region," he said.
The blasts killed at least 35 people and wounded 65 others, Russia's Emergency Situations Ministry said. The casualty tolls were fluctuating immediately after the blasts.
The first explosion occurred at 7:56 a.m. at Lubyanka subway station, killing at least 23 people and injuring 18, the Ministry of Emergency Situations reported on its Web site.
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The Lubyanka station is near the Kremlin and the nation's intelligence service, the Federal Security Service.
Another blast happened about 40 minutes later at Park Kultury station, on the same train line. The ministry reported 12 dead in the second explosion, with 20 more injured.
Three Moscow hospitals were treating the wounded, the ministry said.
Yulia Shapovalova with Russia Today TV was at the second station at the time of the blast.
"The staff members started urgently evacuating people, so that meant they probably knew about the first blast at the Lubyanka station," she said. "All the people -- a huge crowd of people -- slowly started to move. ... As soon as I got upstairs, I heard the blast."
Millions of commuters use the Moscow metro system every day. An estimated 500,000 people were riding trains throughout the capital at the time of the attacks. It was uncertain when the system would return to normal service.
Chechnya launched its fight for independence from Russia in 1991. Thousands have been killed and 500,000 Chechen people have been displaced in their conflict with Moscow.