Meta has been accused of secretly keeping users' deleted Messenger data and sharing it with police by a former employee.
Brennan Lawson, who worked for the social media company after leaving the US Air Force, has filed a legal complaint alleging that Facebook created a tool to access Messenger data that users thought they had deleted.
Lawson, who is suing the company for whistleblower retaliation, alleges he was fired after raising concerns about the tool's legality.
In a filing made to the Superior Court of California in the County of San Mateo, Lawson said he worked for Meta, Facebook's parent company, as a senior risk and response escalations specialist.
"The role required he view extreme content such as beheadings, child rape, and other ruthless and brutal displays of violence or obscenity," the filing states.
Lawson alleges that Meta, founded by Mark Zuckerberg, did not properly protect his mental health, which was affected by his exposure to this content. He is seeking $3m in compensation alongside punitive damages.
Lawson claims to have attended a meeting in late 2018 "where a Facebook manager introduced a new tool to the escalations team".
The filing claims: "Unlike other meetings, there were no materials distributed beforehand for attendees to review.
"This is because, unlike other meetings, Facebook was teaching employees how to utilise a tool that allowed them to circumvent Facebook's normal privacy protocols in order to access user-deleted data.
"This back-end protocol allowed [Lawson's] team to retrieve data in Messenger that users had chosen to delete. Facebook represented to its users that once data was deleted, it was not stored locally and could not be accessed. Not so."
This tool was used by Facebook to ingratiate itself with law enforcement, according to Lawson's claims.
"Law enforcement would ask questions about the suspect's use of the platform, such as who the suspect was messaging, when messages were sent, and even what those messages contained," the legal filing states.
"To keep Facebook in the good graces of the government, the Escalations Team would utilise the back-end protocol to provide answers for the law enforcement agency and then determine how much to share."
Lawson claims to have spoken up during the meeting because he knew it was contrary to Meta's commitments to US regulators about user privacy, and unlawful under data protection regulations in the EU and UK.
He said he subsequently received a critical rating in his performance review and then lost his job - officially for improperly using Facebook's administrator tools to check on his grandmother's account, which she claimed was hacked - as a pretext to firing him because of his whistleblowing.
Meta told Sky News: "These claims are without merit and we will defend ourselves against them vigorously."