New Delhi, Sep 17: The CBI has written to Facebook, Cambridge Analytica and Global Science Research in connection with the illegal personal data harvesting of Indians from the social networking platform, sources said on Monday.

In the letters sent to the companies, the Central Bureau of Investigation (CBI) has sought the details of data collection exercise adapted by them, they said.

After referencing from the Centre, the investigative agency had initiated a preliminary enquiry against Cambridge Analytica and Global Science Research last month over the alleged data theft.

It is alleged that Cambridge Analytica received data from Global Science Research which employed "illegal means" of personal data harvesting of Indians using Facebook, they said. Facebook has over 20 crore users in India.

A preliminary enquiry is the first step to decide whether the allegations deserve a full-blown investigation through an FIR or not, they said. Data mining and analysis firm Cambridge Analytica earlier faced allegations that it used personal information harvested from 87 million Facebook accounts to help Donald Trump win the 2016 US presidential election.

Union Law and IT Minister Ravi Shankar Prasad had told Rajya Sabha in July that the probe will be handed over to the CBI. He had said there were also reports that user data was "illegally accessed by hardware manufacturers who had tie-ups with Facebook. In regard to this issue, Facebook has reported that they are not aware of any misuse of information, including Indian users' information".

The Mark Zuckerberg-owned company had said that the data of about 87 million people -- mostly in the US -- might have been improperly shared with Cambridge Analytica.