(Adds further details on lawsuit, background on litigation)
Dec 4 (Reuters) - Montana has sued OxyContin maker Purdue Pharma LP, withdrawing from a multistate investigation by attorneys general into opioid manufacturers' marketing practices and joining a growing list of states that have broken off to pursue individual lawsuits.
Montana Attorney General Tim Fox announced a lawsuit on Monday accusing Purdue of misrepresenting the likelihood that long-term use of its drug would lead to addiction and falsely claiming it was safe for treating chronic pain.
"Our investigation has revealed and our complaint alleges that executives at Purdue knew exactly what they were doing," Fox said during a news conference.
Purdue did not immediately respond to a request for comment. It has denied similar allegations, saying its medications are approved by the U.S. Food and Drug Administration for long-term use and carry warning labels about their addiction risks.
Opioids were involved in over 33,000 deaths in 2015, according to the U.S. Centers for Disease Control and Prevention. President Donald Trump in October declared the problem a national public health emergency.
State attorneys general have been conducting a multistate investigation into whether companies that manufacture and distribute prescription opioids engaged in any unlawful practices. (Reporting by Nate Raymond in Boston; Editing by Jonathan Oatis)