NEW YORK—American International Group Inc. has begun issuing subpoenas to 400 workers compensation insurance competitors in an attempt to discover possible premium underreporting to states, the insurer said.

A federal judge hearing an ongoing dispute between AIG and several competitors authorized New York-based AIG to engage in “nonparty discovery,” including discovery of practices by insurers not named in the lawsuit, AIG said Monday in a statement.

“Certain competitors continue to wage wasteful litigation against AIG while refusing to reveal how much they and others may have under-reported workers compensation premiums. Given this, AIG has no choice but to issue subpoenas to discover this information and has been given permission by the court to do so,” AIG said in the statement.

The ongoing litigation grew out of a May 2007 lawsuit against AIG originally filed by the National Workers Compensation Reinsurance Pool made up of AIG competitors and operated by Boca Raton, Fla.-based NCCI Holdings Inc. That suit alleged violations of the Racketeer Influenced and Corrupt Organizations Act by AIG, among other assertions.

The pool argued it was excluded from a 2006 settlement in which AIG agreed to pay states more than $343 million to settle allegations that it underreported workers comp premiums over several decades to avoid paying its full share of residual market assessments.

But the federal judge hearing the case in Chicago in August 2009 dismissed the pool as a plaintiff, and AIG became a plaintiff and alleged that several competitors also underpaid residual market assessments to states, AIG said in the statement.

In its lawsuit, AIG alleged competitors such as Liberty Mutual Group Inc., ACE INA Holdings Inc. and Hartford Financial Services Group Inc. engaged in fraud and a conspiracy to harm AIG and hide their misconduct.In a separate but related case, business units of Liberty Mutual Group Inc. have also filed a lawsuit in which AIG is a defendant.

Now AIG is seeking proof through its subpoenas that premium underreporting practices it is alleged to have engaged in were common practice in the industry. Residual market assessments are calculated as a percentage of an insurer’s premiums written in a state.