The states are issuing subpoenas for documents and testimony as part of the probe, but won’t name any companies that are being investigated, Massachusetts Attorney General Maura Healey, whose state is helping to lead the effort, said in a statement on Thursday. Attorneys general in Indiana, New York, Connecticut and Vermont are also part of the bipartisan group.

“The opioid epidemic is a public health crisis that is claiming lives in our state and across the country,” Healey said. “I am working with my colleagues in actively investigating whether manufacturers used illegal practices in the marketing and sale of opioids and worsened this deadly crisis.”

More than 20 U.S. states, counties and cities have sued firms including Johnson & Johnson, Purdue Pharma Inc., and McKesson Corp. in the past year, claiming they fueled a public-health crisis with misleading marketing and aggressive distribution of opioids. Attorneys general in Alaska and Tennessee are also considering lawsuits as their health and legal budgets are stretched to a breaking point by the surge in addictions, overdoses and crime.

Rising Overdoses

Opioid overdoses have quadrupled since 1999, according to the Centers for Disease Control. The states are investigating what role, if any, opioid manufacturers have played in making the epidemic worse. The drugs were involved in more than 33,000 deaths in the U.S. in 2015, up from 19,000 in 2014, the CDC said. In Massachusetts, 2,000 people died last year, a 17 percent increase from the year before, according to Healey’s statement.

Indiana Attorney General Curtis Hill released statistics on overdose deaths in Indiana Thursday showing 1,245 opioid-related deaths in 2015. That’s up nearly 10 percent from 2014.

Hill’s statement also said he wants to “assure Hoosiers that he is taking steps to help address this opioid epidemic in Indiana.”

In Connecticut, the state’s attorney general, George Jespen, said in a separate statement, “The opioid epidemic continues to have a devastating impact in Connecticut. It would be irresponsible to predict at this stage whether our efforts will lead to legal action or relief, but Connecticut residents can be assured that we will pursue this investigation fully.”

Ohio sued five drugmakers in May including J&J, Purdue, Teva Pharmaceutical Industries Ltd., and Allergan Plc, alleging they made false and deceptive statements about the risks and benefits of prescription opioids. Drugmakers and distributors defend the safety of prescription opioids and say they work actively to keep the powerful painkillers from being abused.

The Food and Drug Administration has increased its scrutiny of opioids, directing drugmaker Endo International Plc last week to pull its Opana ER opioid from shelves. Endo said it’s working with the agency to address the request.

“Prosecutors across the country recognize that opioid abuse is a critical issue affecting families everywhere,” Eric Soufer, a spokesman for New York Attorney General Eric Schneiderman, said in an email. “This effort reflects the commitment of a bipartisan group of Attorneys General to bring their combined resources to bear to take a hard look at every facet of this crisis.”