At least one of the two bombs which killed over 30 people in the central Moscow metro system on Monday morning was carried out by a female suicide bomber using plastic explosives, Russia`s top investigator said, according to RIA Novosti.

The first attack took place at 7:52 a.m. (04:52 GMT) at the Lubyanka station, located a short distance from the headquarters of the Federal Security Service (FSB), and killed at least 23 people, with another 39 injured.

The second blast detonated some 40 minutes later at the nearby Park Kultury station, within walking distance of the Kremlin. At least 12 people lost their lives and 23 were injured.

Vladimir Markin told journalists that the second attack had been carried out by a "dark-haired woman" and that "fragments of her body" found at the scene suggested she had had the equivalent of 1.5 kg of TNT strapped to her waist.

A police source earlier told RIA Novosti that "An inspection of the scene indicates that the bomb was detonated at a height of 100-200 cm and was apparently attached to the waist of a female suicide bomber."

Police with sniffer dogs are carrying out inspections of other stations as the capital remains on high alert. Security has been tightened at Moscow`s overland train terminals and airports.

With central Moscow at a standstill, the injured are being ferried to hospitals across the capital by helicopter. Police have appealed for calm.

Emergencies ministry chief Sergei Shoigu has reported to President Dmitry Medvedev on the ongoing operation.

If terrorism is confirmed as the cause of the blasts, this will be the first major terrorist incident in the Russian capital since the autumn of 2004, when 10 people were killed in a suicide bomb attack outside a north Moscow metro station.

The explosion was part of a series of terrorist attacks that also saw 90 people die in two plane bombings and the deaths of over 300 people, many of them children, when Chechen terrorists seized a school in Beslan.

The Moscow metro was last targeted by terrorists in February 2004, when a blast killed some 40 people.

A bomb also hit a Moscow-St. Petersburg express train last November, killing 27 people.

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