BARABOO, Wis. -- An empty pill bottle and letters to and from loved ones have been found by searchers looking for an Illinois teenager whose car was found parked in a highway rest area on Sunday night near the hiking country of the Baraboo bluffs.

Details about the letters were not disclosed, but Sauk County Sheriff Randy Stammen said in a news release that the bottle was for Advil PM, an over-the-counter pain reliever and sleeping aid.

Search teams on Tuesday found a backpack and blankets that belong to Lee Cutler, 18, of Buffalo Grove, Ill. The items were found near the Baraboo River, and his car was found at a wayside next to a bridge over the river.

Cutler was last seen Saturday morning when he dropped off a friend after a Friday night birthday party in his hometown.

A helicopter that uses heat imaging technology joined in the air search of the area Wednesday, but no trace of Cutler was detected, authorities said.