The Federal Trade Commission says it's officially investigating the potential misuse of the personal information of as many as 50 million Facebook users by Trump-connected data analysis firm Cambridge Analytica.

The agency's action comes after it began a probe last week into the social network's admission that it had suspended Cambridge Analytica, which had worked for the Trump presidential campaign, from operating on its platforms while investigating whether it failed to delete information it received through an academic researcher.

At the same time, privacy activists and some in Congress began calling for the agency to investigate whether the improper handling of Facebook users' data violated a consent decree reached with the FTC. In that 2011 decree, Facebook agreed to certain privacy protections to users' data on the network.

Reports in The New York Times and The Observer of London alleged that the data, which included Facebook profiles of more than 50 million users obtained without their permission -- 30 million of them with enough details to match users to other records and build profiles of them -- was used to target voters during the 2016 presidential election.

"The FTC takes very seriously recent press reports raising substantial concerns about the privacy practices of Facebook," said Tom Pahl, the FTC's acting director for its consumer protection bureau, in a statement Monday. "Today, the FTC is confirming that it has an open non-public investigation into these practices.”

Facebook (FB) shares fell 5% to $151.27 after news of the FTC investigation broke.

Facebook shares had already dropped 14% since the scandal broke more than a week ago, eroding Zuckerberg's net worth by $10 billion and the company's market cap by $75 billion.

Facebook CEO Mark Zuckerberg took out ads in Sunday editions of The New York Times, The Wall Street Journal, The Washington Post and six British newspapers to apologize for the recent scandal.

"This was a breach of trust, and I'm sorry we didn't do more at the time," the ads read. "We have a responsibility to protect your information. If we can't, we don't deserve it."

Also on Sunday, the website Ars Technica reported that some Facebook users found the network saved years of contact names, telephone numbers, call lengths and text messages on their Android devices.

In response, Facebook said in a website posting that it does not collect the content of text messages or calls. A spokeswoman told the Associated Press Facebook uses the information to rank Messenger contacts so they are easier to find, and to suggest people to call.