LONDON (AP) — An ex-Russian spy and his daughter lie in critical condition in a British hospital — poisoned, authorities say, with a nerve agent in a targeted murder attempt.

Sergei Skripal, a former Russian agent who confessed to spying for Britain, and his daughter Yulia were found collapsed on a bench in the city of Salisbury on Sunday. A police officer who treated them at the scene is in a serious condition after being exposed to the same substance.

Detectives, intelligence agents and chemical weapons experts are racing to trace the origins of the rare substance and discover who was behind what Home Secretary Amber Rudd called an "outrageous crime" on the streets of a quiet cathedral city.

WHAT IS A NERVE AGENT?

Nerve agents are chemical compounds that block nerve cells from sending messages to each other. Chemical weapons expert Richard Guthrie said they interfere with "the body's dampening-down system."

"It essentially allows nerve signals to get louder and last longer, which prevents proper control of organs," he said.

Nerve agents can come in gas or liquid form, and can be inhaled or ingested through skin contact. They act quickly, inducing vomiting, muscle paralysis, convulsions, respiratory failure, coma, and often death.

WHO CAN PRODUCE NERVE AGENTS?

"This is not a kitchen-sink job," Guthrie said, because nerve agents are relatively difficult to synthesize and manufacturing them requires expertise.

Manufacturing or using nerve agents is prohibited under an international chemical weapons convention signed by all U.N. member states apart from Egypt, Israel, North Korea and South Sudan. The ingredients used to make them are also banned, and are relatively hard to acquire.

That doesn't mean a state was involved, but it would require a well-equipped laboratory and a skilled chemist.

HAVE NERVE AGENTS BEEN USED BEFORE?

Germany developed the first nerve agents before and during World War II, although it did not use them in the war.

Despite global attempts to ban chemical weapons, nerve agents have occasionally been used in war. Saddam Hussein's Iraq used nerve agents and mustard gas during the 1980s Iran-Iraq conflict, and the United Nations has accused the Syrian government of using the nerve agent sarin during the country's seven-year-old civil war. Syria denies it.

Nerve agents have also been used — rarely but terrifyingly — in terrorism and assassinations.

In 1989, South African agents poisoned anti-apartheid activist Frank Chikane by lacing his underwear with a nerve agent. He survived, and a former law-and-order minister in the apartheid government was later convicted of attempted murder.

In 1995 members of doomsday cult Aum Shinrikyo released sarin gas on the Tokyo subway, killing 13 people and injuring about 6,000.

In February 2017, Kim Jong Nam, half-brother of North Korean leader Kim Jong Un, died after falling ill at Kuala Lumpur Airport in Malaysia. Two women have been charged with killing him by applying the banned VX nerve agent to his face.

HOW HARD WILL IT BE TO TRACE THE AGENT IN THIS CASE?

Rudd said the substance used to poison the Skripals was "very rare." Scientists at Porton Down, Britain's high security chemical-weapons research lab, are helping police try to trace its origin.

Andrea Sella, a professor of inorganic chemistry at University College London, said impurities and residues "might provide clues as to the precise chemical process used to manufacture the material." If authorities find the container used to deliver the substance, it would be "a gold mine," he said.

Speculation about the attack has centered on Russia, in part because it echoes the murder of Alexander Litvinenko, a Russian agent turned Kremlin critic who was poisoned with radioactive polonium in London in 2006. A British inquiry concluded that he had been killed on the orders of Russian intelligence, probably with the knowledge of President Vladimir Putin.

The British government said Thursday that it was too early to say who was behind the Skripal attack, but vowed a "robust" response when a perpetrator is identified.

IS THE BRITISH PUBLIC IN DANGER?

England's Chief Medical Officer Sally Davies said Wednesday that there was a "low risk" to the public.

Sella said delivering a nerve agent would be extremely dangerous for the attacker, but there's little risk for the general public "as there is no way of spreading the material around and it would decompose relatively swiftly in damp conditions."

"This was a highly targeted attack," he said.