(AP)—A government watchdog group is calling on the Food and Drug Administration to re-inspect more than a dozen specialty pharmacies with prior records of violations, in light of a recent deadly outbreak tied to compounded drugs.

Contaminated pain injections from a Massachusetts compounding pharmacy have been blamed for that has killed 36 people and sickened more than 500, according to .

In a letter to the FDA sent Thursday, Public Citizen asks the agency to revisit 16 compounding pharmacies that received warning letters from the agency between 2003 and 2009.

Compounding pharmacies traditionally mix customized medications based on doctors' instructions, turning out a small number of specialized products for patients with unusual medical needs. However, some pharmacies have grown into much larger businesses in recent years.