FRANKFORT, Mich. (AP) — Authorities say searches haven’t turned up signs of a small plane that went missing while flying over Lake Michigan.

The Benzie County sheriff’s office says the plane dropped off radar on Sunday night off of northern Michigan, about 4 miles west of Frankfort, and two people were believed to be aboard.

The sheriff’s office says the plane was reportedly having engine trouble and controllers had directed it to an airport in Frankfort when it vanished.

Authorities say the plane took off from Ontonagon in the Upper Peninsula and was heading to Monroe in southeastern Michigan. Crews searched Monday by helicopter, plane and boat, but didn’t find anything.

The U.S. Coast Guard called off its search of Lake Michigan on Monday, but other efforts to find the plane continued.