The Russian men suspected of poisoning Sergei and Yulia Skripal in Salisbury received a phone call after returning to London on the day of the alleged attack, raising the possibility that a backup team played a role in the operation.

One theory being considered by investigators is whether the call, which has not been disclosed before, was a signal to tip them off that the operation had been a success.

Scotland Yard’s counter-terrorism unit said the inquiry into the poisoning was ongoing and detectives would not be drawn on any specifics.

But it is understood that investigators are sifting other pieces of evidence which suggest that the suspects – Anatoliy Chepiga and Alexander Mishkin – may not have been acting alone.

Chepiga and Mishkin flew from Moscow to London on Friday 2 March last year, using the names Alexander Petrov and Ruslan Boshirov.

The pair travelled to Salisbury the following day and returned to the city again on Sunday 4 March.

Timeline Poisoned umbrellas and polonium: Russian-linked UK deaths Show Georgi Markov In one of the most chilling episodes of the cold war, the Bulgarian dissident was poisoned with a specially adapted umbrella on Waterloo Bridge. As he waited for a bus, Markov felt a sharp prick in his leg. The opposition activist, who was an irritant to the communist government of Bulgaria, died three days later. A deadly pellet containing ricin was found in his skin. His unknown assassin is thought to have been from the secret services in Bulgaria. Alexander Litvinenko The fatal poisoning of the former FSB officer sparked an international incident. Litvinenko fell ill after drinking tea laced with radioactive polonium. He met his killers in a bar of the Millennium hotel in Mayfair. The pair were Andrei Lugovoi – a former KGB officer turned businessman, who is now a deputy in Russia’s state Duma – and Dmitry Kovtun, a childhood friend of Lugovoi’s from a Soviet military family. Putin denied all involvement and refused to extradite either of the killers. German Gorbuntsov The exiled Russian banker survived an attempt on his life as he got out of a cab in east London. He was shot four times with a silenced pistol. He had been involved in a bitter dispute with two former business partners. Alexander Perepilichnyy The businessman collapsed while running near his home in Surrey. Traces of a chemical that can be found in the poisonous plant gelsemium were later found in his stomach. Before his death, Perepilichnyy was helping a specialist investment firm uncover a $230m Russian money-laundering operation, a pre-inquest hearing was told. Hermitage Capital Management claimed that Perepilichnyy could have been deliberately killed for helping it uncover the scam involving Russian officials. He may have eaten a popular Russian dish containing the herb sorrel on the day of his death, which could have been poisoned. Boris Berezovsky The exiled billionaire was found hanged in an apparent suicide after he had spent more than decade waging a high-profile media battle against his one-time protege Putin. A coroner recorded an open verdict after hearing conflicting expert evidence about the way he died. A pathologist who conducted a postmortem examination on the businessman’s body said he could not rule out murder. Scot Young An associate of Berezovsky whom he helped to launder money, he was found impaled on railings after he fell from a fourth-floor flat in central London. A coroner ruled that there was insufficient evidence of suicide. But Young, who was sent to prison in January 2013 for repeatedly refusing to reveal his finances during a divorce row, told his partner he was going to jump out of the window moments before he was found.

Skripal poisoning Sergei Skripal and his daughter Yulia were were found unconscious on a bench in the Maltings shopping centre in Salisbury after 'suspected exposure to an unknown substance' which was later identified as chemical weapon novichok. In the aftermath Theresa May blamed Vladimir Putin and expelled 23 Russian diplomats who were suspected of spying. Two Russian men using the identities Ruslan Boshirov and Alexander Petrov were named as suspects. They appeared on Russian TV to protest their innocence.

The Skripals survived. However a local woman, Dawn Sturgess, died after spraying novichok on her wrists from a fake Nina Ricci perfume bottle converted into a dispenser, which had been recovered from a skip by her partner Charlie Rowley.

Detectives say they applied the lethal nerve agent novichok to the door handle of Skripal’s home and caught the train back to London. The Guardian has been told they then returned to their hotel in Bow, east London.

It was there they received an unexplained phone call, a source said.

Shortly after the call, they travelled to Heathrow airport for a 10.30pm flight back to Moscow.

“One theory is that the two waited to hear whether the attack had been a success, and the phone call was the signal to say it had been,” said a source. “It could have been the confirmation that led them to head for the airport.”

It is unclear if the call is linked to another Russian GRU officer, Denis Sergeev, who flew into London from Moscow hours before his two colleagues. He used the cover name Sergey Fedotov.

Sergeev flew back to Moscow on the afternoon of Sunday 4 March. He failed to show up for his booked flight and instead travelled to Russia via Rome.

Investigators are working on the assumption that Skripal was likely under Russian surveillance on Saturday 3 March, when he picked up his daughter Yulia from Heathrow airport and travelled with her in a car back to Salisbury.

Police have also been struggling to make sense of another significant issue.

Detectives have been unable to determine how a bottle of novichok appears to have been found in a bin in Salisbury almost four months after the attack on the Skripals.

The discovery led to the death of Dawn Sturgess, a local woman who sprayed the chemical on her wrists, believing it was perfume.

Police have not been able to establish definitively whether the bottle used by Sturgess was discarded by the GRU officers some 17 weeks earlier, sources say. This is the principal reason why police charged the Russian pair over the attack on the Skripals, but shied away from accusing them over the death of Sturgess.

While ministers have been briefed it is “highly likely” that Sturgess collapsed after being poisoned by the same batch of the chemical, sources admit “some of the details don’t add up”.

“We don’t have all the answers yet,” said a source familiar with the criminal inquiry.

“The theories about the third man, or a second team, cannot be discounted. There is a degree of uncertainty.”

The bottle was discovered by a local man, Charlie Rowley, who gave it to Sturgess as a present. She died from the effects of the chemical days later.

“The bin where the bottle was found was regularly emptied, so it seems inconceivable that it had been there since March,” the source said. “There is also some confusion about whether the bottle was sealed when it was found.

“That is the reason why the CPS could not consider murder charges against the two Russian agents. The police cannot be completely sure the bottle of novichok that poisoned Dawn Sturgess was used against the Skripals.”

In a statement, the Metropolitan police said: “We do not know where the two suspects disposed of the novichok they used to attack the front door of the Skripals’ address, where Dawn and Charlie got the bottle that poisoned them, or if it is the same bottle used in both poisonings, and this remains the case… we don’t know if it will ever be possible to confirm if they are from the same batch.”

Travel records obtained by the investigative website Bellingcat suggest that all three GRU officers were involved in previous operations. In 2014, Sergeev and Mishkin spent eight days together in Prague. In April 2015, Sergeev visited Bulgaria at the same time a Bulgarian arms dealer was mysteriously poisoned.

Mishkin and Chepiga were booked on two sets out flights back to Moscow – the Aeroflot flight they took after their Salisbury weekend, and an identical flight the next day, Monday 5 March. The bookings bolster the theory that the GRU officers went home only after learning their mission had been a success.

The pair spent two nights at the CityStay hotel on Bow Road. Police later discovered tiny traces of novichok in the room where they had stayed.

Both Skripal – a former GRU officer who spied for British intelligence – and his daughter Yulia survived the poisoning. Their whereabouts are unknown. The Russian government has denied “Petrov” and “Boshirov” worked for the GRU. The men appeared on RT, the state TV channel, claiming they were tourists keen on Salisbury cathedral, and denied any involvement in the poisoning.