By Danny Yadron 

Attorneys general in Connecticut and Illinois are investigating Experian PLC, after a Vietnamese man accessed millions of personal records on Americans through one of its affiliates.

It is the first action taken against the data broker and credit bureau since the man, Hieu Minh Ngo, pleaded guilty to running a secret business for fraudsters. Mr. Ngo had access to database of records on some 200 million Americans, according to court filings, though Experian said, in a blog post last month, he likely accessed much fewer.

Representatives for Illinois Attorney General Lisa Madigan and Connecticut Attorney General George Jepsen confirmed the investigation, which was previously reported by Reuters.

Between 2007 and his arrest in Guam in 2013, Mr. Ngo sold credit-card data, social security numbers and other personal information. At some point, he posed as a Singapore private investigator to become a customer of Court Ventures Inc., a California court records firm. The company, acquired by Experian in 2012, allowed its customers to access a large database of information on Americans.

In the blog post last month, Experian said it had no control over the affected database. Rather, it was managed by U.S. Info Search, an Ohio company that had a partnership with Court Ventures. Because of the setup, U.S. Info Search had no knowledge its information was being illegally accessed, said Marc Martin, the company's CEO. Additionally, Experian hasn't provided U.S. Info Search with data on who was affected, Mr. Martin said.

Experian declined to comment on the investigation.

That database was queried more than 3 million times on behalf of Mr. Ngo's customers, according to court records that were first reported by Brian Krebs, a cybersecurity blogger.

The government doesn't know yet how many people were affected.

Ngo faces a sentencing hearing in New Hampshire federal court in June. He is cooperating with the government on at least one other hacking case, according to court records.

Write to Danny Yadron at danny.yadron@wsj.com

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