Former Russian double spy Sergei Skripal and his daughter were exposed to nerve agent before they collapsed in Salisbury.
Police have classified this as attempted murder.
Nerve agents are highly toxic chemicals that can kill people in minutes, and are extremely hard to make.
Experts differ over whether this means Russia did it.
Sergei Skripal, the former Russian double agent, and his daughter Yulia are fighting for their lives in a Salisbury hospital after being exposed to nerve agent.
While officials did not specify the type of nerve agent used, a well-placed source told the BBC it was likely to be extremely rare.
Nerve agents are extremely toxic chemicals that effectively shut down communication between the brain and muscles — in other words, stop the body from working. They are also very hard to make.
Here's what you need to know about the deadly substances.
What are nerve agents?
Nerve agents can take the form of gas, aerosol, or liquid, and enter the body through inhalation, the skin, or the consumption of liquid or food contaminated with them, the Organisation for the Prohibition of Chemical Weapons (OPCW) said.
Symptoms include restlessness, loss of consciousness, wheezing, and a running nose, according to the Centers for Disease Control.
Depending on the amount and method of administration, symptoms can take minutes or hours to occur, Sky News science correspondent Thomas Moore said. When administered in high doses, nerve agents can suffocate victims to death within a couple of minutes, the OPCW said.
It's not clear when the Skripals were exposed to the chemicals and how much was administered to them.
A witness at Zizzi, the restaurant where the Skripals were eating before they collapsed, told the BBC that the elder Skripal "seemed to lose his temper" and "just started screaming at the top of his voice, he wanted his bill and he wanted to go."
Another witness, who saw the stricken Skripals later on, said Yulia "looked like she had passed out" and Sergei "was doing some strange hand movements, looking up to the sky."
What was used on the Skripals?
The type of nerve agent used on the Skripals remains unclear. Investigators have identified it but are not making it public at this point, the BBC reported.
A source close to the investigation told the BBC the nerve agent was likely rarer than sarin gas, which is believed to have been used in the Syrian war and used to kill 13 people in a Tokyo subway in 1995.
The source also said the substance used was not VX, which was used to assassinate the half-brother of North Korean leader Kim Jong Un last year.
The Sun previously reported military scientists on the case as saying the pair might have been poisoned with a "hybrid" kind of thallium, a hard-to-trace heavy metal commonly found in rat poison and insecticides. Detectives originally thought former KGB spy Alexander Litvinenko was poisoned with thallium in London in 2006.
How easy is it to make nerve agents?
The raw materials for nerve agents are relatively inexpensive and easy to procure, the OPCW said. However, the chemical weapon itself is difficult to make.
Victor Madeira, a senior fellow at The Institute for Statecraft who testified to Parliament about Russian covert interference in Britain, told Business Insider: "Nerve agents are rare, tightly-controlled synthetic substances that do require specialized knowledge to manufacture, store and use safely.
"However, that knowledge isn't beyond someone with a good Master's degree in Organic Chemistry, say, and access to a good laboratory. Very difficult, but not impossible."
Chemical weapons expert Richard Guthrie similarly told The Guardian that manufacturing nerve agents require "fairly complicated chemistry," and were "essentially impossible" to make at home.
"Nerve agents, such as sarin or VX, require some fairly complicated chemistry using certain highly reactive chemicals," Guthrie said. "Small quantities could be made in a well-equipped laboratory with an experienced analytical chemist. To carry out the reactions in a domestic kitchen would be essentially impossible."
Does this point to Russia?
Experts appear to differ over whether Russia was responsible.
Matt Tait, a former GCHQ officer, said the method of attack seemed "designed to project that this is a nation-state that’s doing it."
He told The Atlantic: "This is a very extreme form of killing in a way that is designed to project that this is a nation-state that's doing it. Nobody can be under any sort of illusions that this is some sort of run-of-the-mill killing. [...]
"The clear message that they're sending to both people who currently work for their intelligence agencies and also people who used to work for their intelligence agencies … they will make an example of you."
Madeira disagreed. Just because nerve agents are rare doesn't necessarily mean a state actor did it, he said.
"Simply using a 'very rare' nerve agent against Col. Skripal wouldn't necessarily indicate Kremlin (or Russian) involvement," he told BI. "This is why DSTL Porton Down [the UK Ministry of Defence's science lab] and partner agencies are racing to 'fingerprint' the agent used, which will then allow them to narrow the list of potential sources right down."
Rob Wainwright, executive director of Europol, told CNN that attacking an ex-spy with nerve agent in Britain was an "outrageous affront to our security in Europe and our way of life." He warned, however, that people should "exercise caution before jumping to any conclusions."
The Kremlin has vehemently denied any involvement or knowledge of the case.