This timeline shows exactly how Sergei Skripal went from spying for Russia to the centre of an attempted assassination storm in Britain

Sergei Skripal, who's now 66, started passing Russian state secrets to the UK in 1990s. Not much is known about his life before that.

1995 — British spies recruit Skripal while he is a Russian military intelligence officer. He starts passing Russian state secrets to Britain's Secret Intelligence Service, also known as MI6.

Source: BBC

1999 — Skripal leaves the military, having reached the rank of colonel. He keeps passing on state secrets even after his retirement, according to the FSB. He joins the Russian foreign ministry.

Source: BBC, The Guardian

2004 — Skripal is taken into custody near his home in Moscow. He reportedly tells FSB agents after the arrest: "You outplayed me."

Source: BBC, The Guardian

August 2006 — Skripal pleads guilty and is convicted of "high treason in the form of espionage." He is stripped of all his titles and medals, and sentenced to 13 years in jail in Moscow for spying for Britain.

Source: BBC

July 8, 2010 — Skripal is pardoned by then-Russian President Dmitry Medvedev. He and three other spies are released from Russia to the UK and US, in exchange for 10 Russian "sleeper" agents from the US. It is the biggest spy swap between the US and Russia since the Cold War.

Source: The Guardian, Reuters, BBC

July 9, 2010 — The four Russians are flown to Vienna, Austria, for a Cold War-style "spy swap" that lasts 90 minutes. This photo shows the Russian and US planes transporting them, parked side by side on the tarmac.

Source: BBC

Skripal and Igor Sutyagin, another Russian, are flown to a Royal Air Force base in Oxfordshire in a spy exchange with the US. They are kept undercover at a hotel nearby for debriefing.

Source: The Guardian

2011 — Skripal and his wife, Liudmilla, buy a semi-detached home in Salisbury. Skripal's family came to Britain to settle with him after the spy exchange.

October 23, 2012 — Skripal's wife, Liudmilla, dies of cancer. She lived with Sergei in Wiltshire after he was swapped and granted asylum after the spy exchange.

2017 — Skripal’s 43-year-old son dies in St Petersburg, where he is on holiday with his girlfriend. It’s unclear exactly how he died: The BBC said he was rushed to hospital with liver failure, while Sky News said it was a car crash.

Source: BBC

March 4, 2018, 3:47 p.m. — Skripal, now 66, and his 33-year-old daughter Yulia walk around The Maltings shopping centre in Salisbury. Yulia was visiting from Moscow, relatives say.

Source: BBC

See more: CCTV footage shows ex-Russian spy Sergei Skripal on the night he was allegedly poisoned in Britain

4:15 p.m. — A member of the public calls the police after seeing the pair collapsed on a bench at The Maltings.

Source: Wiltshire Police

12 p.m. (approx) — UK Foreign Secretary Boris Johnson says Britain will "respond appropriately and robustly" if evidence emerges of Russia's involvement in Skripal's suspected poisoning.

See more: Boris Johnson promises to 'bring Russia to heel' following the alleged poisoning of Sergei Skripal

2:11 p.m. — The UK's national counterterrorism police say they are leading the investigation into the suspected poisoning. London's Metropolitan Police says it has not been declared a terrorist incident and that it was keeping an "open mind as to what happened."

See more: UK counter-terror police are now leading the investigation into the suspected poisoning of a former Russian spy

As of Tuesday evening, Sergei and Yulia Skripal remain in critical condition.