Who targeted a former Russian spy and his daughter in Salisbury - and why remain unclear.
Investigators have been working around the clock since Sergei and Yulia Skripal were found collapsed in a shopping centre on 4 March.
Here are some of the key questions at the centre of the investigation:
The Prime Minister has confirmed the pair were hit with Novichok, which refers to a string of chemical weapons developed in Russia starting in the 1970s. She said it was "highly likely" Russia was involved.
Theresa May said the substance must either have been used in a state attack, or have got into the hands of an individual, in which case Russia had "lost control" of the agent.
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.
At least 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, where the Skripals were found.
:: 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.
Many residents feel they are not being given enough information quickly enough to understand what is going on.
But officials have insisted that the risk to the public is low.
:: 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, a nuclear arms expert. 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.
Former ambassador 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."
:: What will happen next?
Mrs May has said Russia must explain by the end of Tuesday why a nerve agent they created was used in Britain.
This response will be considered by the Government on Wednesday, when she will give a statement on the range of options available.
These could include expelling diplomats and Russian intelligence agents and maybe even the Russian ambassador, too.
She may also seek to hurt Mr Putin and his circle where it hurts - financially. Russian oligarchs famously use London as a playground to spend and park their money.