A counter terrorism police chief has described how hundreds of investigators and scientists painstakingly worked to trace “ground zero” – the spot where the former Russian spy Sergei Skripal and his daughter Yulia were poisoned with a nerve agent.

Commander Dominic Murphy, head of the Met police’s counter terrorism command, told an inquiry into the novichok poisonings that a restaurant, pub and a car were suspected before the door handle of his home was pinpointed as the source of the poisoning almost two weeks after the Skripals fell ill.

Murphy said that up to 250 counter terrorism police were sent to Salisbury during the investigation and told the inquiry a “whole series” of officers would have touched the door handle before it was confirmed as “ground zero”.

Skripal and his daughter, Yulia, fell ill on a bench in Salisbury city centre on Sunday 4 March 2018. They survived but four months later a Wiltshire woman, Dawn Sturgess, fell ill after her partner, Charlie Rowley, found a fake perfume bottle containing novichok.

Murphy said that from Tuesday 6 March police knew that the Skripals had suffered nerve agent poisoning. He said: “It focuses the mind quite substantially. I would describe it as a very sobering moment to realise there was a chemical warfare agent present in Salisbury. We were putting police officers in scenes that were contaminated with one of the most dangerous substances on the planet.”

A priority became finding out where the Skripals were first contaminated, both to work out what had happened to them and to protect the public. The officers nicknamed this “ground zero”. Murphy said this was an “unfortunate term” because of its connotations with the 9/11 attacks but it stuck.

“From the very start one of the important things about finding ground zero was giving us an evidential start-point for our investigation into who might be responsible for this,” he said. “But clearly we needed to find if there was any novichok remaining in Salisbury and it remained a total focus of the investigation.”

The investigative strategy was to work backwards from the bench in the Maltings shopping centre where the Skripals fell ill. “Ground Zero moved a few times,” said Murphy.

An early theory put forward by a public health expert was that they had ingested the nerve agent in the nearby Zizzi restaurant, where they ate lunch, which would have led to them collapsing just two minutes later.

Within a few days, traces of nerve agent had been found in the pub they drank in before visiting Zizzi, suggesting they had been exposed to poison earlier and undermining the two minute theory.

By the end of the week the highest readings yet were found in Skripal’s BMW, which he and Yulia had travelled to the city centre in. “This was a very significant milestone,” said Murphy. The BMW was thought likely to be ground zero.

But attention was also being focused on Skripal’s address and traces of nerve agent were found inside. Murphy said Skripal’s cat was euthanised because it was likely to have been contaminated while his two guinea pigs died of natural causes.

Government scientists told police they believed the nerve agent in the house had not been planted there but had been brought in by police officers and the focus turned to the front door handle.

On 17 March tests proved there were “very high” levels of novichok on the door handles even though it had rained and snowed and a “whole series” of investigators had touched it. The handle was designated ground zero.

The inquiry continues.