The London offices of Cambridge Analytica have been searched by enforcement officers from the UK's information commissioner.
The High Court granted the data watchdog a warrant amid claims the firm amassed information about millions of people without their consent, based on a 2014 quiz on Facebook.
The seven-hour search finished in the early hours of Saturday.
Both Cambridge Analytica and Facebook deny any wrongdoing.
A group of people, some wearing ICO enforcement jackets, entered the building housing Cambridge Analytica's London headquarters at 20:00 GMT on Friday - less than an hour after a high court judge granted the warrant.
Several hours later members of the Information Commissioner's Office were seen leaving the offices and a van - thought to be carrying gathered evidence - was driven away from the rear of the building.
The ICO applied for the warrant to access the databases and servers of Cambridge Analytica.
The search is part of a wider investigation into political campaigning.