WASHINGTON — A federal probe into Facebook's sharing of user data with Cambridge Analytica now involves the FBI, the Securities and Exchange Commission and the Justice Department, the Washington Post reported.

Representatives from these agencies have joined the Federal Trade Commission in the inquiry, the newspaper reported, citing five unnamed people familiar with the matter. Those people spoke on condition of anonymity because the probes are not complete.

The probe reportedly centers on what Facebook knew in 2015, when it learned that the political data-mining firm Cambridge Analytica had improperly accessed the personal data of tens of millions of Facebook users. Facebook didn't disclose the incident with the political firm, which later worked for the Trump campaign and other Republican candidates, until this March.

The Post said the probe will look at why Facebook didn't inform users or investors earlier. Investigators are examining actions and statements of the company and its executives, including CEO Mark Zuckerberg, according to the report.

Facebook confirmed to The Associated Press that it has received questions from these agencies and said it is cooperating with the inquiry.

Representatives from the FBI and Justice Department did not immediately respond to a request for comment. An SEC spokesperson declined to comment. The FTC said in March it was looking into whether Facebook engaged in unfair acts that hurt consumers, and which might have violated a consent decree it reached with the FTC in 2011.

Facebook shares fell 1.2 percent in after-hours trading following the report Monday, after rising 1.6 percent in the regular session.