LONDON — British police have identified two suspects in the poisoning of former Russian double agent Sergei Skripal and his daughter Yulia, a source with knowledge of the investigation told CNN Thursday.

The source said investigators have been combing through months of surveillance camera footage from UK airports and from Salisbury, where the Skripals were poisoned in an attack the British government blames on Russia.

Using facial recognition technology, authorities discovered two “fresh identities” — individuals not known to have been spies or used in other attacks — the source added.

Investigators have crosschecked that information with the manifest of the commercial flight on which the suspects were believed to have left Britain soon after the attack in March. Those names were aliases, the source said, and police have not yet revealed them.

Britain’s Press Association news agency also reported Thursday that police had identified suspects in the case. London’s Metropolitan Police declined to comment.

The attack led to a diplomatic dispute with Russia. The UK expelled 23 Russian diplomats and more than 20 other countries ,including the United States, followed suit.

Russia has repeatedly denied any involvement in the attack and has put forward multiple theories as to who may have been behind tt.

The Skripals were found unconscious on a bench in the city of Salisbury on March 4. They were treated for exposure to the rare Soviet-era military-grade nerve agent Novichok and discharged separately from hospital several weeks later. Police believe they came into contact with the substance via the door handle of Sergei Skripal’s house.

Tensions escalated further earlier this month when two British residents of Amesbury, a town eight miles north of Salisbury, fell ill after being exposed to the same nerve agent. Dawn Sturgess, 44, later died, while her partner Charlie Rowley, 45, remains in hospital.

Although police said there was no suggestion that the couple had been deliberately targeted, a murder investigation was launched. Last week, police announced that they had identified a small bottle found in Rowley’s home as the source of the nerve agent that killed his partner.

It is unclear where Rowley found the bottle, or whether it is the same batch of nerve agent that poisoned the Skripals.

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