WASHINGTON - The US Federal Trade Commission (FTC) is probing Facebook over whether it violated terms of a consent decree over its use of personal data, according to Bloomberg News citing a person familiar with the matter.

The investigation involved whether the internet giant once allowed Cambridge Analytica to use some data of Facebook users without their knowledge, said the person, who asked not to be identified because the investigation is confidential.

Cambridge Analytica is a British data analysis company in activities allegedly connected with US President Donald Trump during his 2016 campaign.

Facebook is allegedly violating the FTC consent decree when it allowed an app developer working for Cambridge Analytica to gain access to personal information.

Facebook "reject any suggestion of violation of the consent decree" in a statement Saturday.

Facebook's stocks incurred the biggest one-day drop in four years by 7 percent on Monday and the slump continued on Tuesday, falling by about 5 percent by 13:00 Eastern Standard Time.