Russian Prime Minister Vladimir Putin says investigators¹ should view catching² the organisers of Monday's suicide attacks as a "matter of honour".

俄罗斯总理弗拉基米尔·普京称，相关调查人员应当把抓获星期一地铁自杀式爆炸袭击的策划者当成一项“荣耀”。

He said the security services, who have been widely criticised in the media, should drag the bombing masterminds "from the bottom of the sewers³（排水沟，下水道） ".

On Tuesday, Russians observed a day of mourning for 39 people killed in the attacks on two Moscow Metro⁴ trains.

Officials have blamed the bombings on militants⁵（激进分子） from the North Caucasus.

Tension remains⁶

Russian media reports are linking the attacks to the death of a rebel leader from Ingushetia - Alexander Tikhomirov, also called Said Buryatsky.

He was killed by Russian security forces earlier this month after being blamed for an attack on a train last year in which 26 people died.

Russian newspaper Kommersant quoted investigation⁷ sources as saying they believed Tikhomirov had recruited（招募，雇佣） 30 suicide attackers from Ingushetia and neighbouring Chechnya.

Police fear that two of the bombers⁸ he trained might have blown up the Moscow trains to avenge⁹ his death, according to reports.

The BBC's Richard Galpin in Moscow says the main concern for Russia is whether Monday's bombings were the start of a new wave of attacks by rebels.

Security has been stepped up in the capital, as police are reported to be searching for three people sighted with the bombers.

The Metro is back up and running, but commuters say the city was not yet back to normal.

"I feel the tension on the Metro. Nobody's smiling or laughing," university student Alina Tsaritova told the Associated Press news agency.

Paying respects

The authorities say two female suicide bombers detonated explosives packed with pieces of metal.

Russian newspapers have published pictures of two dead women, purported¹⁰ to be the suicide attackers.

In the early 2000s, Russian cities were hit by attacks by women from the Caucasus known as "black widows" because their husbands were believed to have been killed by Russian soldiers.

Russian media have also published a CCTV image they say is a man suspected by police of helping¹¹ in the attacks.

In a meeting with senior officials, Mr Putin urged investigators to find the organisers.

"We know they're lying low, but it's a matter of honour for law enforcement bodies to scrape（刮，擦） them from the bottom of the sewers and into the daylight," he said.

Much of Mr Putin's political reputation was built on his tough stance（姿势，立场） against rebels from Chechnya while he was president.

His successor as president, Dmitry Medvedev, said the government would consider revamping（修补，修改） anti-terrorism laws to try to prevent further attacks.