A search for a missing aircraft that was carrying two Kalispell Daily Inter Lake staff writers and two men from Missoula shifted sharply late Monday from the Flathead Valley area to Sanders County and the National Bison Range.

A command post for the search was set up Monday night at the National Bison Range Visitor Center, far from the original search area that extended from Kalispell as far as Glacier National Park and Canada.

The plane had been missing since Sunday afternoon.

Sgt. Ernie Freebury of the Flathead County Sheriff's Office noted that the terrain where the search now is focused is considerably less rugged and more open than Glacier Park.

He said the Bison Range area "is a whole lot more survivable than it would be somewhere in the park."

Around 20 members of Lake County Search and Rescue launched a ground search just after 9 p.m. on the west side of the Bison Range, which is about 80 miles south of Kalispell.

Lake County Sheriff Lucky Larson said the search would go on all night.

Witnesses in the area reportedly observed a low-flying blue-and-white plane around 4 or 4:30 p.m. Sunday.

The 1968 Piper Arrow, a blue-and-white single-engine airplane with a low wing, had taken off from Kalispell City Airport just after 1:30 p.m. Sunday.

The plane had been rented by Sonny Kless, 25, in Missoula and did not return to either Missoula or Kalispell airports.

On board were Melissa Weaver, 23, a police and courts reporter with the Inter Lake, and Erika Hoefer, 27, a business reporter and page designer who also writes for the Flathead Business Journal.

The other man on board was Brian Williams of Missoula, according to the Flathead County Sheriff's Office. They were heading out on an aerial sightseeing tour that included Glacier Park.

"They were on their day off and just having fun," said Daily Inter Lake Publisher Rick Weaver, who is not related to Melissa Weaver. "We're just hoping for the absolute best. We hope that they're found and everybody's OK."

Late Monday afternoon, Freebury said Federal Aviation Administration radar data had been analyzed and it had been determined that the plane traveled north along the Whitefish Range, entered Glacier National Park airspace heading south, then flew along the Swan Mountain Range, across Flathead Lake and over Polson and Ronan.

Freebury said the plane last was tracked about 300 feet above ground level west of the National Bison Range at Moiese.

Debbie Alke, administrator of the Montana Aeronautics Division, said the search was shifted to Sanders County, with both the Lake County and Sanders County sheriff's offices as well as search-and-rescue groups involved.

Weaver's car still was at the Kalispell City Airport on Monday morning; she and Hoefer were supposed to be at a barbecue with friends at 6 p.m. Sunday.

Their absence and lack of phone contact was considered strange by Weaver's roommate, prompting her to contact the Flathead County Sheriff's Office and other agencies Monday morning.

Initially, Sheriff Mike Meehan said the group was going on a scenic flight, mostly likely in the Glacier National Park area.

Freebury organized an initial search involving a state airplane checking on prominent wilderness airstrips just after noon.

By Monday afternoon, several private aircraft with Red Eagle Aviation and the Montana Civil Air Patrol were involved in the search, along with a U.S. Customs Service helicopter that was detailed to the Western Governors Association conference in Whitefish.

Glacier National Park officials and the Montana Aeronautics Division also are engaged in the search.

The pilot, Kless, last made radio contact with the tower at Glacier Park International Airport at 2:11 p.m., reporting that he was east of Kalispell, traveling south to north. It was unclear whether any flight plan had been left at any airport.

Freebury said cell-phone tower information also was analyzed.

Meehan said Hoefer last updated her Facebook page at 1:40 p.m. with a message reading, "We're flying to the park and we're later going to a barbecue."

The last voice message from any of the individuals' phones occurred at 1:51 p.m.

However, there was text messaging between Weaver and Hoefer at 3:47 p.m. A sheriff's detective pursued a subpoena on Monday to access that information from a cell-phone company server.

"The last text they sent pinged off the Ronan tower," Freebury said. "That was our second confirmation, besides the radar data, that they were in that area."

Freebury said the content of the messages was not noteworthy.

"They were just conversational texts, nothing significant at all," he said.

Meehan said the type of plane involved was carrying an older-style transponder and bore the tail number N-7581J. The plane is registered to Joel Woodruff of Stevensville.

"It is our understanding that it's an old [transponder], and you would have to fly almost directly over it to make contact, and that's a hindrance," Meehan said.

On Monday evening, Alke said no transponder signal had yet been picked up west of the Bison Range.

Weaver, a University of Montana graduate from Billings, and Hoefer, from Beloit, Wis., both began work at the Inter Lake in December 2009.

Weaver is the daughter of Daniel and Katherine Weaver of Billings; Hoefer is the daughter of William and Candy Hoefer of Beloit.

Kless rented the plane from Northstar Aeronautics at Missoula International Airport, the Missoulian newspaper reported.

Before the plane took off Sunday afternoon, Kless told Northstar the plane would arrive back in Missoula after its scheduled 4 p.m. return, according to Jeff Woodruff, Northstar general manager.

Kless "wasn't sure when" he would return or how late he would be, Woodruff said.

He said he first became aware the plane was missing Monday morning. He said Kless had been flying that particular single-engine Piper Arrow for about a year.

The Associated Press and Sasha Goldstein of the Lake County Leader contributed to this story.

Reporter Jim Mann may be reached at 758-4407 or by e-mail at jmann@dailyinterlake.com.