It was a scene of horror: a bench, two unconscious individuals, a furtive poison. The attempted murder in early March of Sergei Skripal and his daughter, Yulia, caused headlines around the world. Skripal was a former officer with Russia’s GRU military intelligence agency. He arrived in the UK in 2010 as part of a spy swap. Yes, he had given secrets to MI6. But why try to kill him eight years on? And who applied novichok to his front door in Salisbury?

Over the past nine months, lurid details of an audacious Moscow murder plot have emerged. Immediately, Theresa May blamed Vladimir Putin and expelled 23 Russian diplomats who were suspected of spying. The Kremlin denied involvement and said it was the victim of “hostile action”.

Behind the scenes, officers led by Scotland Yard sifted through thousands of hours of CCTV footage. By May, they had made a provisional breakthrough. They identified two Russian men – Ruslan Boshirov and Alexander Petrov – who visited Salisbury during the weekend, not once but twice. The pair flew in from Moscow to Gatwick airport with genuine Russian passports and visas. The suspicion, however, was that they were undercover operatives travelling under assumed names. Hitmen, in fact, sent to the UK on a one-off mission.

By summer, detectives had gathered sufficient evidence to consider charging the two men with murder. Police had found the weapon: a fake Nina Ricci perfume bottle converted into a dispenser. It was specially made to apply the poison, gunked out on to the handle of Skripal’s door. The discovery was made under tragic circumstances. A local woman, Dawn Sturgess, inadvertently sprayed novichok on her wrists. She died in hospital. Her partner, Charlie Rowley, had apparently found the bottle discarded in a skip. He fell ill but survived.

Meanwhile, police tracked down the budget hotel in Bow, east London, where Boshirov and Petrov stayed. They found minute traces of novichok, a deadly nerve agent developed during the late Soviet era in state labs. The plot was reminiscent of the 2006 murder of Alexander Litvinenko, who was killed with radioactive green tea.

In September, the prime minister laid out the evidence – or some of it – to MPs. There were compelling images of the killers. They were recorded arriving at Salisbury train station, walking in the direction of Skripal’s home and returning afterwards – visibly relieved, with Petrov grinning.

Back in Moscow, the pair appeared on Russian state TV. Petrov said friends had suggested they visit Salisbury – a “wonderful town”. Boshirov added that they were attracted by the city’s cathedral and spire. Their performance was awkwardly ridiculous and led to widespread online mockery.

Bellingcat, the investigative website, uncovered further compelling facts. In late September, it established Boshirov’s true identity. His real name was Colonel Anatoliy Chepiga. He was a serving officer with the GRU, Skripal’s old service. He had received Russia’s highest state award in 2014, possibly for his role in Russia’s take over of Crimea.

Soon afterwards, Bellingcat named Petrov as Alexander Mishkin. He was also a career GRU spy – a medical doctor who trained at a military academy in St Petersburg. He received a new identify and passport some time after joining the GRU and moving to Moscow. At Mishkin’s home in Loyga, a muddy village in northern Russia, his proud grandmother kept a photograph of her grandson, reporters were told. It showed Mishkin receiving the same outstanding award as Chepiga: hero of Russia.

The person bestowing the award and shaking hands with Mishkin was well known in Russia and familiar to anyone who watched the evening news – Putin. In November, the Kremlin announced sad news concerning the GRU boss, Igor Korobov. He had died, the Kremlin said, after a “long and grave illness”.