SAN FRANCISCO, March 19 (Xinhua) -- The world's largest social media company Facebook said Monday that it has hired a digital forensics firm, Stroz Friedberg, to investigate the alleged leakage of user data abused for the U.S. presidential campaign in 2016.

Facebook has been mired in a dispute over a possible data breach after the data of more than 50 million users of its service was said to be inappropriately used by a British data analysis company, Cambridge Analytica, in activities allegedly connected with U.S. President Donald Trump during his 2016 campaign.

The dispute incurred the biggest one-day drop of Facebook's stocks by 7 percent in four years Monday.

"Independent forensic auditors from Stroz Friedberg were on site at Cambridge Analytica's London office this evening," Facebook said in a statement.

The auditing was launched at the request of Briton's Information Commissioner's Office, which has announced it is pursuing a warrant to conduct its own on-site investigation, Facebook said.

It noted that Cambridge Analytica had agreed to comply and afford the firm complete access to their servers and systems.

"This is part of a comprehensive internal and external review that we are conducting to determine the accuracy of the claims that the Facebook data in question still exists," Facebook said.

California-based The Mercury News daily reported Monday that Facebook data was used by Trump's campaign during the primaries but not during the general election.

U.S. Federal Election Commission numbers showed the firm collected 5.9 million U.S. dollars in 2016 from Trump's campaign, the daily said.

The Associated Press quoted Facebook as saying last week that Cambridge Analytica received user data from a Facebook app years ago that purported to be a psychological research tool, though the firm wasn't authorized to have that information.

Facebook admitted that an estimated 270,000 people had downloaded the app and shared their personal details with it.

Last Friday, Facebook said in an official post that it had suspended "Strategic Communication Laboratories (SCL), including their political data analytics firm, Cambridge Analytica."

It said the two companies had failed to delete user data acquired in 2015 in violation of Facebook rules.

Last Saturday, Facebook said that the claim of a data breach is completely false, arguing that people knowingly provided their information.

"No systems were infiltrated, and no passwords or sensitive pieces of information were stolen or hacked," Facebook said in its official blog post.

Facebook asserted Monday that Cambridge Analytica had told it that the data accessed had been destroyed.

"If this data still exists, it would be a grave violation of Facebook's policies and an unacceptable violation of trust and the commitments these groups made," Facebook said.