The poison used to target a former spy and his daughter is Russian-made nerve agent novichok, the PM has said. The chemicals are among the most deadly nerve agents ever created.
:: Who made it?
There are more than 100 formulations in the novichok family, all developed by the Soviet Union in the 1970s and 1980s.
Novichok is the Russian word for "newcomer".
:: How did it get here?
Andy Oppenheimer, a biological and chemical specialist, has told Sky News the chemical could have been brought into the UK through an airport.
"These things can evade detection if they are very small amounts and very well shielded, whether they are biological, chemical or a biological agent," he says.
"This chemical may have come through an airport, we really don't know yet."
:: How is it used?
Novichok agents are dispersed as an ultra-fine powder rather than a gas or vapour.
They can be inhaled, ingested, or absorbed through the skin.
:: What are the symptoms?
As with other nerve agents, the chemicals block acetylcholinesterase, an enzyme active in the nervous system.
The result is involuntary contraction of all muscles, leading to cardiac arrest and asphyxiation.
:: Is washing clothing enough?
Dr Jenny Harries, deputy medical director at Public Health England, has said the "immediate risk to the general public remains low".
She advised people who were in either The Mill pub or Zizzi restaurant in Salisbury on Sunday March 4 to wash any clothing they were wearing, wipe personal items including phones or handbags with baby wipes and to wash hands thoroughly.
Mr Oppenheimer says the advice was just a "precaution".
He told Sky News: "Unlike a conventional explosive device going off, it really has a kind of a spread, it will produce particular symptoms.
"I would have thought those symptoms would have emerged by now and all the advice is a precaution."
:: Is there an antidote?
All nerve agents have an antidote in atrophine, but it needs to be administered as soon as possible.
The problem is it can take time to diagnose that a nerve agent has been used.
:: Does it contravene international law?
Novichok is "highly illegal", "extremely powerful" and created to avoid detection, says Mr Oppenheimer.
Significantly, several of the novichok chemicals are so-called binary weapons – with two non-toxic precursor chemicals that can be mixed prior to use.
This makes them safer to transport and handle.
One of these is novichok-5, derived from the highly potent A-230 chemical. Its precursor chemicals are ordinary organophosphate pesticides and can be legally made at agricultural chemical manufacturers.
According to Dr Vil Mirzayanov, who worked on the novichok programme in the Soviet Union until he fled to the US, the legitimate use of precursor chemicals was deliberate as it meant they were not placed on the controlled list of the Chemical Weapons Convention.
:: Who could be behind it?
Theresa May told the Commons her Government has concluded it is "highly likely" Russia is responsible for the poisoning of Sergei Skripal and his daughter Yulia.
The PM said the attack, using a "military-grade" nerve agent, fits a pattern of Russian aggression and said Russia's ambassador to the UK has been summoned to explain what happened.
She said: "We will not tolerate such a brazen attempt to murder innocent civilians on our soil."
Foreign Secretary Boris Johnson has told the Russian ambassador that Moscow must "immediately provide full and complete disclosure" of its novichok nerve agent programme to the Organisation for the Prohibition of Chemical Weapons.
Moscow has been given until the end of Tuesday to respond, Mrs May said.
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If Russian involvement is proved, it will be considered an "unlawful use of force by the Russian state against the United Kingdom", she added.
Mr Skripal, 66, and his daughter Yulia, 33, remain in a critical condition more than a week after they were found slumped unconscious in Salisbury.