Bob Nelson's cell phone rings but no one answers. Family members and authorities still call it, hoping it will lead to him.

Nelson, 60, of Battle Lake, Minn., and his single-engine Piper Cherokee have been missing since Thursday evening, after authorities say he took off in a storm.

Nelson's wife, Mollie Crawford, spoke with her husband on the cell phone minutes before he took off.

"He had been waiting on the ground because of bad weather," she said. "He said a window was clearing and he would have the opportunity to get in the air.

"He wouldn't have gone if he thought the weather was bad," she said.

Heavy rain, hail and funnel clouds moved across parts of North Dakota and Minnesota on Thursday. Nelson was reported missing about 8:30 p.m. The last radar tracking signal from Nelson was southwest of Jamestown, about an hour after he left.

Nelson had an "emergency locate" transmitter on the plane, his wife said. She said it was "baffling" that the transmitter was not emitting any signals.

"He took meticulous care of his airplane - it was his hobby and his passion," Crawford said. "He would have been sure that the ELT was working before he ever took off."

Crawford asked help Tuesday from landowners in the Dakotas and southwest Minnesota to search their property for signs of her husband and his airplane.

"What I'm hoping is that farmers will be kind enough to look at shelterbelts, ravines or those areas that might not be visible from the air," she said. "My family would be grateful."

Air and ground searches have turned up no trace of Nelson. Authorities have been trying to track Nelson's cell phone, but they say a voice connection has to be made before a location can be traced.

"Yes, it's still ringing," Crawford said Tuesday night, choking back tears. "We have been working with Verizon and it doesn't appear they have been able to get a triangulation fix on his location."

Authorities on Tuesday expanded their search in North Dakota and into northern South Dakota for the missing pilot and his airplane.

Air and ground searches have covered every square mile of North Dakota south of Interstate 94, from Bismarck to Fergus Falls, Minn., where Nelson has a hangar, said Rick Robinson, a spokesman for the North Dakota Department of Emergency Services.