A subsidiary of consumer credit history provider Experian Plc is being probed by a number of US states over a data breach exposing the social security numbers of about 200 million US citizens to potential criminal activity.

Reuters, citing Maura Possley, a spokeswoman for Illinois Attorney General Lisa Madigan, reported that a multi-state investigation is going on into the subsidiary.

Jaclyn Falkowski, spokeswoman for Connecticut Attorney General George Jepsen, told the news agency that the state is looking into the matter as well.

A spokesman for Experian told Reuters that the company does not comment on such investigations as a matter of policy.

London-listed Experian is a constituent of the FTSE 100 index. It explains itself as a global information services company, providing data and analytical tools to clients across the globe. It helps businesses to manage credit risk, prevent fraud, target marketing offers and automate decision making, according to its website.

In March, a Vietnamese man pleaded guilty in New Hampshire federal court to managing an underground website that offered personal data of Americans including social security numbers. The data could be used for identity theft and other criminal activities.

The man named as Hieu Minh Ngo obtained some of the data through a US firm known as Court Ventures, which was bought by Experian in March 2012. The firm also has a data-share agreement with a firm known as U.S. Info Search to provide clients with access to a database of social security numbers of around 200 million Americans.

Ngo obtained an account with Court Ventures by posing as a Singapore-based private investigator, according to court documents. Ngo is now awaiting sentencing in New Hampshire federal court.

Making use of Court Ventures, Ngo's customers made some 3.1 million queries of the U.S. Info Search database over an 18-month period ending in February 2013. Authorities are yet to ascertain the number of people whose data was accessed illegally.

"We are actively pursuing the facts and we are working to help uncover what records may have been affected," Experian spokesman Gerry Tschopp told Reuters.

Experian earlier said in a blog that Ngo obtained access to U.S. Info Search data through Court Ventures prior to the time it acquired the company.

"After Experian's acquisition of Court Ventures, the U.S. Secret Service notified us that Court Ventures had been and was continuing to resell data from a U.S. Info Search database to a third party, possibly engaged in illegal activity," the company said.