(Reuters) - A bipartisan group of state attorneys general announced on Thursday that they are jointly investigating the marketing and sales practices of drug companies that manufacture opioid painkillers at the center of a national addiction epidemic.

Attorneys general from states including Massachusetts, Texas, Illinois and Pennsylvania announced the investigation two weeks after Ohio Attorney General Mike DeWine sued five drug manufacturers for misrepresenting the risks of opioids.

“We are looking into what role, if any, marketing and related practices might have played in the increasing prescription and use of these powerful and addictive drugs,” District of Columbia Attorney General Karl Racine, a Democrat, said in a statement.

It was unclear exactly how many states are involved in the probe, though officials said a majority of attorneys general are part of the coalition. Among those leading the probe is Tennessee Attorney General Herbert Slatery, a Republican.

Officials did not specify which companies were under investigation.

Opioid drugs, including prescription painkillers and heroin, killed more than 33,000 people in the United States in 2015, more than any year on record, according to the U.S. Centers for Disease Control and Prevention.

Separate lawsuits by attorneys general in Ohio and Mississippi, the two states that have pursued opioid-related cases to date, have targeted Purdue Pharma LP, Johnson & Johnson, Endo International Plc, Teva Pharmaceutical Industries Ltd and Allergan Plc.

Teva in a statement said on Thursday it is “committed to the appropriate promotion and use of opioids.” Representatives for the other companies did not immediately respond to requests for comment.

The companies have denied wrongdoing, saying the U.S. Food and Drug Administration approved their products as safe and effective and saying that they carried warning labels that disclosed their risks.

In announcing his office’s lawsuit on May 31, Ohio Attorney General DeWine said the drug companies helped unleash the crisis by spending millions of dollars marketing and promoting such drugs as Purdue’s OxyContin.

The lawsuit said the drug companies disseminated misleading statements about the risks and benefits of opioids as part of a marketing scheme aimed at persuading doctors and patients that drugs should be used for chronic rather than short-term pain.

Similar lawsuits have been filed by local governments, including two California counties, the cities of Chicago, Illinois and Dayton, Ohio, three Tennessee district attorneys, and nine New York counties.