Police have moved steel barriers into place at one of the crime scenes in Salisbury as they said the investigation into the nerve agent attack could last until the summer.

Scotland Yard’s counter-terrorism chief, Neil Basu, said he was confident that the culprits who left the Russian former spy Sergei Skripal and his daughter, Yulia, critically ill would be found but that the inquiry would be prolonged.

Asked if the focus was on Skripal’s BMW, following speculation that it may have been sabotaged, Basu said: “Our focus is on the movements of the Skripals. We are open minded and will follow that evidence wherever it takes us.”

The notion that the investigation could last until the summer at least was supported by the arrival of steel barriers behind the Mill pub, one of the places the Skripals visited before they fell ill.

Elsewhere, the lingering concern about the risk to public health was illustrated by the refusal of an Oxfam shop in Salisbury to take donations.

A spokesperson for Oxfam said: “Some of our volunteers in Salisbury were concerned that donated clothes could potentially be contaminated. We respect their concerns and so have suspended donations. We hope to accept donations again soon when more advice and information about the case is available.”

Public Health England is expected to update its advice to people who were in the Mill and the Zizzi restaurant that the Skripals visited on 4 March within the next few days.

A police officer stands next to screening surrounding the Zizzi restaurant visited by Sergei Skripal and his daughter, Yulia Photograph: Peter Nicholls/Reuters

Scientists are still trying to work out the best way for people to deal with clothes that need to be dry-cleaned. It is thought there are concerns about how any trace of the nerve agent might interact with chemicals used in dry-cleaning.

The authorities have contacted 131 people who could have been exposed to the nerve agent.

A woman who was in the Zizzi restaurant with her boyfriend and newborn baby a few hours after the Skripals were taken ill has said she was surprised it took nearly two weeks for her to be approached by the police.

She said: “We were spoken to on the 15th, which was 11 days later. Responding to people’s worries and getting information out there was really crap, really slow.

“I was told [by the police] that we should have no worries about our health, as if we were affected we would have seen something by now. I was really worried because I have a small baby.”

A man has been charged in connection with an incident in which police officers manning the cordons in Salisbury were allegedly abused and assaulted.

Kim Rogerson, 56, of Salisbury, was arrested on Monday afternoon in Castle Street, the location of Zizzi. He has been charged with assaulting a police officer, assaulting a police staff member, being drunk and disorderly in a public place and a racially aggravated public order offence. He will appear before magistrates next month.