The Tunisia beach massacre is linked to a previous terror attack in the country, British police believe.
There is "strong" evidence of a connection between the killings in Sousse in June and a shooting at the National Bardo Museum outside the capital Tunis three months earlier, Scotland Yard said.
It is the first time UK police, who have been supporting the investigation into the more recent atrocity, have officially linked the two events.
Commander Richard Walton said: "We are now linking evidentially the Bardo Museum investigation with the Sousse investigation."
In the aftermath of the Sousse attack, Tunisian officials said gunman Seifeddine Rezgui trained in neighbouring Libya at the same time as two attackers who targeted the Bardo museum in March, killing 22 people, including a British woman.
Mr Walton, head of Met's counter terrorism command, declined to provide details of the connection, only saying: "It is strong evidence that links the two."
He said a team of officers are working closely with the Tunisian authorities on both inquiries and the coroner in the UK has been told about the link.
Terror group Islamic State claimed responsibility for the Sousse attack, in which 30 Britons were among 38 tourists killed.
Scotland Yard provided an update on the massive investigation launched in the wake of the beach murders.
Tunisian authorities have now arrested 150 people in relation to the killings, with 15 of those charged with terrorism offences.
Mr Walton said Tunisian authorities have deployed "significant resource" into the investigation.
They have carried out operations across the country to disrupt terrorist activity and identify suspects, he said.
Those charged face allegations including being involved in a terrorist plot and providing logistical or other support.
A trial in relation to the murders is not expected to take place for up to 18 months.
British officers have taken a total of 459 witness statements since the day of the beach attack in June.
A number of people have referred to a second gunman wearing red shorts, but investigators said that although they retain an open mind, they now believe the person identified was a beach security worker who attempted to stop the gunman.
More than 370 photograph and video files from mobile devices are being assessed by detectives, while specialist advisers have been sent to Tunisia as authorities in the country review security at resorts and tourist attractions.
Mr Walton also disclosed that Rezgui's body has not been claimed due to the shame his family feel and fear of reprisals if they do so.