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 (ICO) 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.
Information Commissioner Elizabeth Denham has said she was looking at whether personal data was acquired in "an unauthorised way", whether there was sufficient consent to share the data, what was done to safeguard it and whether Facebook acted robustly when it found out about the loss of the data.
Cambridge Analytica's acting chief executive, Alexander Tayler, said the company has been in touch with the Information Commissioner's Office (ICO) since February 2017 and it remained committed to helping the investigation.
In a statement, he said checks in 2015 showed all the Facebook data had been deleted but the company was now undertaking an independent third-party audit to verify none remained.
Cambridge Analytica chief executive Alexander Nix was suspended on Tuesday after footage broadcast on Channel 4 appeared to show him suggesting tactics his company could use to discredit politicians online.
Claims over whether Cambridge Analytica used the personal data of millions of Facebook users to sway the outcome of the US 2016 presidential election and the UK Brexit referendum have also been raised.
The company denies any of the data harvested in the 2014 Facebook quiz created by an academic was used in its work for Donald Trump's campaign.
Meanwhile, the director of Vote Leave has denied allegations of links between his campaign and Cambridge Analytica. Dominic Cummings said claims by the Observer newspaper are "factually wrong, hopelessly confused, or nonsensical".
In a separate development, Brittany Kaiser, Cambridge Analytica's former business development director, has told the Guardian the firm carried out data analysis for Leave.EU, the rival Brexit campaign to Vote Leave that was fronted by Nigel Farage.
Cambridge Analytica said it did "no paid or unpaid work" for Leave.EU.