SUN PRAIRIE, Wis. -

"Every day this goes on, everyone gets a little sadder and feels a sense of helplessness," said Matt Dayton about his missing nephew, who's from Sun Prairie.

Jonathan Norton, his fiancee Amber Smith and three of her family members were on aboard a plane that disappeared Sunday in the central Idaho back country.

Rescue crews are trying to hone in on a weak transmitter signal and they're increasing the number of people on the ground.

But while Dayton is hopeful, he said it doesn't look good.

Amber's father, Dale Smith, was piloting the plane, but around 1:30 p.m. that day, he reported engine trouble. He lost radio contact then, but radar saw the plane drop 10,000 feet in a minute. Temperatures have been about 15 below zero overnight in Yellow Pine, the nearest town.

On Tuesday, a weak emergency locator transmitter signal was picked up about 1 mile south of a nearby airstrip, but the signal may be misleading due to the mountainous terrain.

Dayton joined the search efforts Tuesday.

"The thing about Jonathan, he would give you the shirt off his back to help anyone out," Dayton said. "He was just a real nice kid."

Twenty-four-year-old Norton grew up in Sun Prairie and went to the Mormon Church there, where his family still attends. He met his fiancee, Amber Smith, at Brigham Young University-Idaho. The two accounting majors are supposed to get married Jan. 4.

"The sad part was, on Monday, I got Jonathan and Amber's wedding invitation in the mail," Dayton said.

The couple was on the plane flying from Baker City, Ore., to Butte, Mont., after visiting Smith's family over Thanksgiving break.

The number of agencies and personnel involved in the search has increased daily, with more than 90 people working Thursday.

Five planes from the Civil Air Patrol and two Idaho Army National Guard helicopters are also being used to grid the entire search area.

The Civil Air Patrol is using forward-looking infrared radar as an added capability to search efforts. This specialized equipment, brought in from Wyoming, detects ground temperatures and can pick up anomalies, such as sunlight reflecting off metal.

Dayton didn't know how long the search for the family would last.

"We hope we can find them safely and the good thing is, at least, they're together," said Dayton, who said Amber's father is an experienced pilot who owned the plane.