Russian security services were hunting for more "Black Widows” yesterday amid fears that the women who blew themselves up in the Moscow Metro were part of a 30-strong suicide squad trained by a Chechen terrorist leader.

Agents from the Federal Security Service believe that the women were avenging the death of Said Buryatsky, the leading ideologue of the Islamist rebels in the North Caucasus, who was killed this month in Ingushetia. They were trying to establish whether the attack was a one-off response to his death or the start of a suicide bombing campaign that he had prepared before the FSB tracked him down.

Moscow observed a day of mourning as a woman who died in hospital became the bombers’ 39th victim. Police circulated grisly photographs of the facial remains of the Black Widows in an attempt to identify the women who blew themselves up on trains at Lubyanka station, next to the FSB’s headquarters, and at Park Kultury.

Vladimir Putin, the Prime Minister, urged security chiefs to drag out “from the bottom of the sewers” those who had planned the bombings, in language that recalled his 1999 pledge to pursue terrorists and "waste them in the outhouse." He said: "It's a matter of honor for law enforcement bodies to scrape them from the bottom of the sewers and into the daylight."

Medvedev has threatened to destroy the people behind Moscow’s worst terrorist attack for six years. But he also urged officials to improve living conditions in the North Caucasus to draw people away from extremism.

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"People want a normal and decent life, no matter where they live," he said.

Ramzan Kadyrov, Chechnya’s Kremlin-backed President, offered a characteristic response in the newspaper Izvestia, writing: "We believe that terrorists must be hunted down; they must be poisoned like rats."

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