WASHINGTON — Anti-addiction activists are calling on a Senate committee to release findings from a 3-year-old investigation into links between painkiller manufacturers and nonprofit medical groups.

In a letter Wednesday, a coalition of groups asks the Senate Finance Committee to make public information from the probe, which was launched in May 2012. At the time, former Senate Finance Chairman Max Baucus, D-Mont., and Sen. Charles Grassley, R-Iowa, sent letters to three drug manufacturers asking about their collaboration and financial ties to seven medical groups. The drugmakers were Purdue Pharma, Endo Pharmaceuticals and Johnson & Johnson.

The Senate has not publicly released any documents or findings from the investigation.

The request comes as deaths linked to addictive painkillers like Vicodin and OxyContin have reached an estimated 16,000 per year, more than heroin and cocaine combined. The Centers for Disease Control and Prevention has called on doctors to limit their use of the medications to the most serious forms of pain, such as cancer patients and end-of-life care. But the vast majority of prescriptions written in the U.S. are for more common ailments like arthritis and back pain.

The letter was signed by National Coalition Against Prescription Drug Abuse and 35 other groups.