The investigation into the attempted murder of Russian double agent Sergei Skripal has widened, as British police sealed off the graves of his wife, Liudmila, and son Alexander and confirmed that 21 people had been treated since the incident.

Anxious residents accused authorities of keeping them in the dark after nearly 200 troops trained in chemical warfare arrived in Salisbury in an escalation of the nerve agent investigation.

Specialists from the RAF and Royal Marines began removing contaminated materials to be sent for analysis at the nearby Porton Down defence laboratory.

Yesterday it emerged that Colonel Sergei Skripal may have been poisoned at his home, meaning the nerve agent could have been in his system for hours before he eventually collapsed in the city centre a week ago.

Chemical weapons experts said this could mean that a new strain of nerve agent — which was slower to take effect — may have been used to target the spy and his daughter, Yulia.

Detective Sergeant Nick Bailey, made seriously ill by the nerve agent, is now thought to have been contaminated at Skripal's home, having been one of the first officers into the house.

Yesterday the investigation widened as officers began to focus on the areas Skripal and his daughter are known to have visited before they collapsed.

It is understood that the Government expects analysis from Porton Down to confirm the attack originated in Russia.

The Home Secretary was to host a Cobra security meeting overnight, NZ time. And, should the analysis point to the Russians, British Prime Minister Theresa May is expected to announce sanctions against the country as soon as tomorrow.

Yesterday, in scenes likened to a disaster movie, a military convoy carrying 180 trained chemical warfare troops entered the Wiltshire city, although authorities insisted there remained no threat to public health.

At the same time, police officers wearing full chemical protection suits to shield them from exposure to the nerve agent were stationed at the Salisbury cemetery where Skripal's wife and his son's ashes are buried. It is understood they took away trinkets and flowers, which were placed by Skripal and his daughter last Sunday.

A military convoy of a dozen vehicles arrived at Salisbury District Hospital to remove a police car and other contaminated material from the site.

While police reiterated there was no need for alarm, residents said they were fearful, questioning the need for the full chemical protection suits if there was no danger.

David Bayfield, 76, who runs Salisbury Family Butcher stall close to the bench where the Skripals collapsed, said: "We are fed up being kept in the dark.

"Everyone is saying it's fine, but there are police everywhere and now the army. How can it be fine?

"We know he went to the pub but where else did he go? Who did he brush past? Who did he touch? I don't think we've been told the whole truth."

One of Skripal's neighbours, whose home is within the police cordon, said authorities had made no attempt to give them any information to allay their fears.

The Metropolitan Police's counter-terrorism unit, which is leading the investigation, said it had "requested assistance from the military to remove a number of vehicles and objects from the scene in Salisbury town centre as they have the necessary capability and expertise".

Both Skripal, a senior officer in the Russian military intelligence who was convicted of spying for MI6, and his daughter remain critically ill. He came to Britain in 2010 in a spy exchange.

Counter-terror police are now investigating whether Yulia Skripal, 33, inadvertently brought a gift for her father from Moscow that contained the nerve agent, planted by Russian intelligence in her luggage.