DULUTH — Police have arrested 30 people in an alleged crack cocaine trafficking ring in Duluth and Superior, Wis.

The arrests came this week after a nine-month investigation dubbed “Operation Crackdown.”

Investigators say suspects were trading crack cocaine for government food-assistance debit cards at about half the face value of the cards. Investigators say they used decoy cards to conduct controlled buys of crack cocaine.

Lt. Steve Stracek, commander of the Lake Superior Drug and Violent Crime Task Force, said alleged dealers would either go to the store with the buyer and buy groceries with the cards, or they would force the buyer to prove the card still had money, and then take the card.

Authorities are seeking another 10 suspects.