A former worker at Meta is being investigated by police over suspicions he downloaded about 30,000 private Facebook images.
The engineer is alleged to have designed a programme to access the pictures while avoiding internal security checks, UK court documents show.
A specialist detective from the Metropolitan Police's cybercrime unit is investigating.
Meta confirmed the suspected breach was discovered more than a year ago and said it referred the matter to police itself.
It said affected users had been notified and the worker was sacked. Its security systems have also been upgraded.
The engineer, who lives in London, is on bail while the investigation continues. A request to vary his bail conditions was recently granted by Highbury Magistrates' Court.
"Protecting user data is our top priority," said a Meta spokesperson.
"After discovering improper access by an employee over a year ago, we immediately terminated the individual, notified users, referred the matter to law enforcement and enhanced our security measures.
"We are co-operating with the ongoing investigation."
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Meta - along with Google - last month suffered a landmark court defeat in the US after being found liable for a woman's social media addiction.
A jury in Los Angeles found Instagram, which is owned by Meta, and YouTube, which is owned by Google, were responsible for harm caused to the 20-year-old - awarding her $6m in damages.
Meta and Google both said they disagreed with the verdict and plan to appeal.
The decision is seen as a bellwether that will inform hundreds more cases against social media companies for creating addictive algorithms.
(c) Sky News 2026: Former Facebook worker investigated by police over download of 30,000 private images
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