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(CNN) —  

Around 180 British troops have been deployed to the English city of Salisbury to aid in the investigation of a nerve agent attack on a former Russian double agent and his daughter.

Personnel from the Royal Air Force, British Army and Royal Navy were among those sent to Salisbury on Friday, a Ministry of Defence spokesman told CNN.

London’s Metropolitan Police tweeted that the public should not be alarmed and the public health advice remained the same. It explained that the counterterrorism division had requested assistance from the military to “remove a number of vehicles and objects from the scene.”

A split image showing former Russian spy Sergei Skripal and his daughter, Yulia Skripal.
Misha Japaridze/AP/Yulia Skripal/Facebook via AP
A split image showing former Russian spy Sergei Skripal and his daughter, Yulia Skripal.

Former Russian spy Sergei Skripal, 66, and his daughter Yulia, 33, were still hospitalized in “very serious condition” after being poisoned earlier this week, UK Home Secretary Amber Rudd said as she visited Salisbury on Friday.

The pair – believed by authorities to have been deliberately targeted – were found slumped on a bench near a shopping center on Sunday afternoon.

Rudd visited several of the sites cordoned off by investigators before heading to Salisbury District Hospital.

Detective Sgt. Nick Bailey, a policeman who also fell ill, is in serious condition, but is “conversing and engaging” with visitors, Rudd added.

Authorities earlier revealed 21 people had received medical attention in the aftermath of the incident, but only three people were still being treated Friday.

Jack Taylor/Getty Images
Military personnel, pictured arriving at Salisbury District Hospital on Friday, were deployed to help remove potentially contaminated vehicles and help gather evidence.
Jack Taylor/Getty Images
Military personnel, pictured arriving at Salisbury District Hospital on Friday, were deployed to help remove potentially contaminated vehicles and help gather evidence.

Kier Pritchard, temporary chief constable of Wiltshire police, which covers Salisbury, told Britain’s Sky News on Thursday that “multiple” police officers and members of the public had been assessed by medics: “A number of those had been through the hospital treatment process, there have been blood tests and they’re having treatment in terms of support and advice.”

Identifying the source of the nerve agent continues to be central to the probe but so far, Rudd has refused to name any potential culprits.

“We will have to wait until we’re absolutely clear what the consequences could be, and what the actual source of this nerve agent has been,” Rudd said. “At the moment, our priority is going to be the incident, which is why I’m here.”

Western intelligence services consider Russia a leading suspect based on previous attacks that used a similar substance and method, a Western intelligence official told CNN. The official cautioned that the investigation was still in its early stages.

If a Moscow link was proved, it would plunge relations between the West and Russia to a new low, and would call into question the British government’s ability to protect UK residents at home.

UK Home Secretary Amber Rudd is shown a scene connected to the nerve agent attack on Friday morning.
Jack Taylor/Getty Images
UK Home Secretary Amber Rudd is shown a scene connected to the nerve agent attack on Friday morning.

On Friday, Russian Foreign Ministry Sergey Lavrov said Russia was ready to assist “any investigation” but that it was “not necessary to hurl unfounded accusations on TV.”

Earlier this week, a spokesman for the Russian embassy in London described comments by UK Foreign Secretary Boris Johnson – who vowed a “robust response” in the event that state involvement was proved – as “strongly anti-Russian.” The spokesman said Johnson’s comments were an attempt to politicize the affair, and attacked the media for aiding the UK government’s efforts.

Police say they know the nerve agent used in the attack, but have declined to say what it was or how they suspect it was administered. They called on anyone who visited the area on Sunday, including a branch of the Zizzi restaurant chain or the nearby Bishop’s Mill pub, to come forward with any information that might help them piece together what happened.

Related: In Salisbury, grist for conspiracy theorists, and an elusive truth

Spy swap

Skripal is considered a traitor in his native Russia after a 2006 conviction for spying for the UK, for which he was sentenced to 13 years in prison.

He arrived in the UK in 2010 as part of a spy swap between the United States and Russia, when the two countries exchanged agents on chartered planes on a runway at an airport in Vienna, Austria.

According to previous reports by Russia’s state news agency RIA Novosti, Skripal had spied for Britain as an intelligence officer during the 1990s and he had continued to communicate with British foreign intelligence agency MI6 after his retirement in 1999.

Skripal’s daughter is thought to be one of the few members of his immediate family still alive after his wife, Lyudmila, and son Alexander died in recent years. Police have sealed off both graves, the UK’s Press Association reported, but it was unclear on Friday why.

The grave of Alexander Skripal, son of ex-Russian spy Sergei Skripal, on March 8.
Matt Cardy/Getty Images
The grave of Alexander Skripal, son of ex-Russian spy Sergei Skripal, on March 8.

Litvinenko’s wife: Russians in UK ‘feel unsafe’

On Tuesday, Boris Johnson said there were “echoes” in this case of what happened to former Russian spy Alexander Litvinenko, who died a slow death after drinking tea laced with highly radioactive polonium-210 in 2006 in a hotel in the Mayfair section of London.

A detailed UK inquiry later concluded that Russian President Vladimir Putin probably approved the operation by Russian agents to kill Litvinenko. At the time, the Russian Foreign Ministry dismissed the UK investigation as politically motivated.

Speaking in Berlin on Thursday, Litvinenko’s window, Marina, told CNN there was a “similarity of a way to assassinate” between the attack on Skripal and the case of her husband.

Marina Litvinenko said Russians living in the UK feel “insecure and very unsafe” and that the UK government should do more to protect those who have claimed political asylum.

CNN’s Laura Smith-Spark and Euan McKirdy contributed to this report.