Over 250 members of the UK counterterrorism police were working on the investigation into the poisoning of the Russian-born double agent Sergei Skripal. The probe was proceeding with "speed and professionalism," according to British Home Secretary Amber Rudd.

The police identified over 200 witnesses and were looking into over 240 pieces of evidence linked to the case, Rudd told reporters after chairing a meeting of the UK government's Cobra emergency committee on Saturday.

However, she said it was "too early" to apportion blame for the attack on the 66-year-old Skripal and his daughter in the south English town of Salisbury.

"The two victims remain in hospital and they're critical but stable," Rudd told reporters.

Forensic team at Skripal family graves

Police detective Nick Bailey also fell ill after the attack last Sunday. He is still hospitalized, but he has recovered enough to sit up and engage with his family, according to Rudd.

The authorities said a nerve agent was used in the attack. Also, the UK government deployed around 180 soldiers, including chemical warfare experts, to various locations around Salisbury. A local restaurant and a pub remain closed, and forensic teams were collecting evidence from Skripal's home.

Forensic experts were also active at the Salisbury cemetery where Skripal's wife and son were buried. Skripal's wife passed away from cancer in 2012 at the age of 60. His son died last year, at the age of 43, reportedly while in St. Petersburg, Russia. Officers were seen setting up a tent over the son's memorial stone before apparently placing items in several yellow barrels.

London to respond against Moscow?

Many have speculated that Russia was behind the poisoning of Skripal, who served as an officer of the Russian military intelligence service in the mid-1990s when he was recruited by British MI6 spies.

Skripal was arrested in Russia in 2006 and convicted of "high treason." However, he was delivered to the UK in 2010 after a spy swap.

British papers reported that the London government was mulling over retaliatory measures against Russia in case the theory of Moscow's involvement is confirmed. UK Prime Minister Theresa May could announce sanctions "as soon as Monday," according to the Daily Telegraph.

dj/sms (AP, AFP, Reuters)