The Civil Air Patrol launched one last air search Saturday for a missing Minnesota pilot, after scouring more than 30,000 square miles in three states.

"We have a few airplanes up today," Col. Karl Altenburg, with the Civil Air Patrol office in Fargo, said early in the afternoon. "After today we will suspend air operations, and we may or may not keep the search open unless we obtain leads."

Bob Nelson, 60, of Battle Lake, Minn., and his single-engine Piper Cherokee have been missing since Aug. 24, after he took off in a storm. The plane left the Bismarck airport and was en route to Fargo or Fergus Falls, Minn., where Nelson had a hangar, authorities said.

Searchers on Saturday flew over an area near Harvey, in central North Dakota, where the Air Force said it received a single signal from an emergency locater transmitter on the day Nelson and his plane vanished. ELTs are designed to emit a continuous signal. The search came up empty, and it was called off abut 6:30 p.m., Altenburg said.

Nelson had an ELT on the plane but authorities have not picked up any signals from it, authorities said. The Aug. 24 signal may have come from another source, Altenburg said.

The Air Force Rescue Coordination Center in Langley, Va., is coordinating the CAP's search, Altenburg said. The decision to halt the search is made jointly by the Civil Air Patrol, the North Dakota Department of Emergency Services and the Air Force, he said.

The cost of the search is estimated at about $55,000, which is funded by the Air Force, Altenburg said. Pilots have logged 580 hours, combing North Dakota, southwest Minnesota and northeast South Dakota.

No trace of Nelson or his airplane has been found.