Washington DC's top prosecutor has filed a lawsuit against Facebook over its involvement in the Cambridge Analytica scandal.
The lawsuit, filed on Wednesday by Karl Racine, the attorney general for the District of Columbia, marks the first major effort by US regulators to punish the firm after the data-mining company improperly accessed data from as many as 87 million users.
It alleges that Facebook misled users about the security of their data and failed to properly monitor third-party apps.
In 2013, a researcher launched a Facebook app, promoted as a personality quiz, which claimed to generate a personality profile.
The app also collected the personal information of users' Facebook friends and the information was eventually sold to Cambridge Analytica, a political consulting firm.
After the revelations about Cambridge Analytica, congressional hearings were held and Facebook changed what sorts of data it lets outside developers access.
"We're seeking to hold Facebook accountable for jeopardising and exposing the personal information of tens of millions of its users," Mr Racine said.
"We hope this lawsuit will ensure Facebook takes better care with its data."
The lawsuit alleges that Facebook told users that it would protect their personal information, but allowed the app developer to collect and sell the data of users who had not downloaded or used the app.
It also alleges that Facebook was aware in 2014 that the developer wanted to download the information about users' friends but "failed to monitor or audit the app".
The information of more than 340,000 District of Columbia residents was exposed but only 860 downloaded the quiz, Mr Racine said.
Facebook has already produced "reams of documents" in response to the attorney general's investigation, officials said.
After the suit was filed Wednesday, Facebook said in a statement: "We're reviewing the complaint and look forward to continuing our discussions with attorneys general in DC and elsewhere."
Earlier this year, Facebook was fined £500,000 in the UK for its involvement in the scandal.
Additional reporting by Press Association.
