LONDON (Reuters) - Former Russian double agent Sergei Skripal remains critically ill in hospital more than a month after he was poisoned with a nerve agent in the southern English city of Salisbury.

Police officers guard the cordoned off area around the home of former Russian intelligence officer Sergei Skripal in Salisbury, Britain, April 3, 2018. REUTERS/Hannah McKay

This is what we know - and don’t know - so far:

WHAT HAPPENED?

At 1615 GMT on March 4, police received a call from a member of the public about two people who were acting strangely. The police found Skripal, 66, and his daughter, Yulia, 33, unconscious on a bench outside The Maltings shopping center in Salisbury. The pair were taken to Salisbury District Hospital.

A police officer, Detective Sergeant Nick Bailey, was one of the first to respond to the incident. He was taken to hospital and later released.

Britain said more than 130 people were affected by the attack. More than 50, including three children, reported to hospital.

WHAT POISONED THEM?

On March 12, British Prime Minister Theresa May said the couple had been poisoned with a military-grade nerve agent from the Novichok group of poisons developed by the Soviet Union in the 1970s and 80s.

There are several variants of Novichok, a binary weapon containing two less toxic chemicals that when mixed react to produce a poison several times more lethal than sarin or VX.

Russia’s ambassador to Britain, Alexander Yakovenko, has identified the alleged poison as Novichok A-234, derived from an earlier version known as A-232.

May said her government had concluded that it was “highly likely that Russia was responsible” for the poisoning or that it had lost control over some of the nerve agent.

Russia has denied involvement and Russian officials have suggested the British secret services, possibly with U.S. help, poisoned the Skripals to stoke anti-Russian hysteria.

Russian President Vladimir Putin has said it is nonsense to think that Moscow would have poisoned Skripal and his daughter.

“Russia does not have such (nerve) agents,” Putin said on March 18. British Foreign Secretary Boris Johnson said it was overwhelmingly likely that Putin himself made the decision to attack Skripal, an accusation the Kremlin said was shocking.

DIPLOMATIC FALLOUT

May gave Russia a day to provide an explanation but none came. On March 14, she ordered 23 Russians who she said were spies working under diplomatic cover to leave Britain.

On the eve of its March 18 presidential election, Russia ordered 23 British diplomats out of Moscow. It also shut down the activities of the British Council, which fosters cultural links, and Britain’s consulate-general in St Petersburg.

The leaders of the United States, Germany, France and Germany condemned the first known use of a military-grade nerve agent in Europe since World War Two, saying it threatened the security of the West.

In the biggest Western expulsion of Russian diplomats since the height of the Cold War, the United States ordered out 60 Russians and Moscow responded by expelling 60 U.S. diplomats. Other Western countries then expelled about 70 Russians and the Russians largely followed suit.

For a graphic of the expulsions: here

HOW DO WE KNOW IT WAS NOVICHOK?

May said Britain’s military laboratory at Porton Down had identified Novichok, though the government has not said how it knows it came from Russia.

Russia has questioned on what basis Britain has made the conclusion that the poison originated in Russia and whether Britain has a reference sample of Novichok.

To give an exact match, chemical weapons experts said Britain would have to compare it to a sample of Russian-made Novichok. It is unclear how Britain would have such a sample.

Other ways of identifying the source would be British or Western intelligence information. None has been made public.

Gary Aitkenhead, chief executive of the Defence Science and Technology Laboratory at Porton Down, said on April 3 it was unable to say whether the nerve agent used was produced in Russia.

But he said the substance required “extremely sophisticated methods to create, something only in the capabilities of a state actor.”

Britain has asked inspectors from the Organisation for the Prohibition of Chemical Weapons to test the poison. The results have not been published.

WHAT IS NOVICHOK?

Most public knowledge about Novichok comes from two Russian scientists who went public about the nerve agent shortly after the Soviet Union collapsed in 1991. It was developed by the Soviet Union’s military in the 1970s and 80s by the state chemical research laboratory.

Vil Mirzayanov, a Russian chemist who helped develop the poison, told Reuters last month that only Russia could have carried out the attack.

He said Russia maintains tight control over its Novichok stockpile and that the agent is too complicated for a non-state actor to have weaponized.

Another Cold-War-era scientist, Professor Leonid Rink, told Russia’s RIA news agency the attack did not look like Moscow’s work because the Skripals had not died immediately.

The Soviet Union’s chemical weapons programme was in disarray after the Cold War. Some toxic substances and know-how could have got into the hands of criminals.

WHERE DID THE POISONING HAPPEN?

Dean Haydon, Britain’s’ senior national coordinator for counter-terrorism policing, said on March 28 a nerve toxin had been left on the front door of the Skripals’ home in England.

Yulia Skripal arrived in Britain from Russia at London’s Heathrow Airport at about 1440 GMT on March 3. At about 1340 GMT on March 4, the Skripals arrived at the car park of the Sainsbury’s supermarket store at The Maltings shopping center.

They went to the Bishop’s Mill pub and then to Zizzi, an Italian restaurant, at about 1420 GMT. They remained there until about 1535 GMT. A member of the public alerted the emergency services at about 1615 GMT.

Yulia was in a critical condition for nearly four weeks but on March 29 doctors said her condition had improved, though she is still being treated in hospital.

Her father remained in a critical but stable condition, the hospital said. A British judge said last month that the Skripals might have suffered permanent brain damage.