What nerve agent was used to poison a former Russian spy and his daughter in Salisbury and who carried it out remain a mystery.
Investigators have been working round the clock since the pair were found collapsed in a shopping centre last Sunday amid intense speculation over possible Russian involvement.
Here are the key questions at the centre of the investigation:
Speculation over responsibility for the murder has focused on Russia – and the Prime Minister is reportedly close to naming the state as prime suspect.
The Times newspaper on Monday cited government sources as saying police and security services have established enough evidence to link Moscow to the attempted killings.
Chair of the foreign affairs select committee Tom Tugendhat said the poisoning looks like state sponsored attempted murder.
But Moscow has denied the suggestions, saying the allegations are part of the “demonisation” of Russia.
Because Mr Skripal had been pardoned and released in a swap, attempting to kill him would be a brazen move – “against all rules” according to Russia expert Andrei Soldatov.
Five locations in Salisbury have been identified as potential sites of contamination.
They include Zizzi restaurant, the cemetery where Mr Skripal’s wife and son are buried, The Mill pub and Mr Skripal’s home.
Investigators have accounted for some of the movements of Mr Skripal and his daughter. It is thought they were at home before heading to the Zizzi restaurant in Castle Street, where they were seen at 2.30pm.
Just over an hour later they are believed to have been caught on CCTV from a camera at Snap Fitness 24/7, walking through an alleyway that connects Zizzi and an area of Maltings shopping centre.
They were found slumped on a bench there at around 4.15 pm.
:: What was used
Police announced on Wednesday that they believe Mr Skripal and his daughter were targeted with a nerve agent.
The substance is usually administered in liquid form, and can be sprayed into the body through the eyes and skin or put into food and drink and ingested.
Nerve agents work by interrupting the messaging from the brain to the muscles – which can become paralysed, causing death by asphyxiation.
There are several different kinds of nerve agent, including Sarin and VX nerve agent – believed to have killed Kim Jong Un’s half brother in Kuala Lumpur last year.
:: Is there a further risk?
Traces of the nerve agent used against Mr Skripal and his daughter have been found at The Mill pub and Zizzi restaurant in Salisbury.
Up to 500 people are thought to have visited there between Sunday lunchtime and Monday night.
They have been told to wash their clothes and clean items such as mobile phones, wallets and jewellery, in case of contamination. The possessions and work clothes of staff at Zizzi’s restaurant have been removed and burned.
But officials have insisted that the risk to the public is low.
“I am confident this has not harmed the health of anyone who was in The Mill pub or Zizzi’s restaurant,” chief medical officer Professor Dame Sally Davies said.
She described guidance to clean belongings as “precautionary advice aimed at only those people who were at the venues at these times, which I believe to be below 500 people.”
:: Why Skripal?
Sergei Skripal was sentenced in Russia to 13 years in jail in 2006, for giving information about Russian agents to British intelligence.
He was released in 2010 as part of a high-profile spy swap between Russia and the US, along with three other Russians.
But we know little of his life in Salsibury.
He kept a low profile compared to others who were swapped at the same time, including Igor Sutyagin. He said Mr Skripal had “talked about his family”, adding: “It seemed to me it was his family which was his major joy.”
The profile of the former spy has led to speculation over why he might have been targeted.
But Sir Christopher Meyer told Sky News there are some “strange things” about the case.
“Did he continue to work against Russia?” he said. “Because he might have done something, or been with some British intelligence agency, say, which was such that it enraged people in the Kremlin.”