FARGO, N.D. (AP) _ Police say they believe some party houses are using scanners to monitor radio traffic and get advance warning about impending raids.

Over the weekend, scanners likely were used to tip off partygoers in at least three cases before officers arrived, said Dan Griffin, a sergeant with the Moorhead, Minn., police department.

Griffin said police are responding by using scrambled radio frequencies and mobile computers not easily monitored by the public.

Using scanners to monitor police activity ``may work once or twice, but it's not a foolproof method,'' Griffin said.