The search for a plane missing since Sunday with four people on board shifted somewhat Tuesday toward the lower Flathead River west of the National Bison Range.

By midafternoon, at least four boats fitted with sonar equipment were grid-searching the lower Flathead River between the town of Dixon and the river's confluence with the Clark Fork River.

Search officials stressed that an extensive aerial search continues, too, over a much broader area.

"Who knows? In just a matter of minutes, it could be miles and miles from the river," said Carey Cooley, a public information officer for the Lake County Sheriff's Office.

One of the missing people, Brian Williams, is a native of Coeur d'Alene.

Lake County Sheriff Lucky Larson said roughly 100 people are involved in the official search, plus many friends and family members of those on the plane are searching as well.

The focus on the river is because of eyewitness accounts.

"The one thing that has been consistent is that they have seen a blue-and-white aircraft flying low over the Flathead River," Cooley said.

The 1968 Piper Arrow, a blue-and-white single-engine plane, last was tracked by radar around 300 feet above ground level west of the Bison Range at Moiese.

It went off the radar screen at 4:02 p.m., Flathead County Sheriff's Sgt. Ernie Freebury said.

Federal Aviation Administration radar data showed the airplane had traveled from Kalispell north along the Whitefish Range, entered Glacier National Park airspace, then headed south along the Swan Mountain Range and flew across Flathead Lake to the Bison Range, Freebury said Monday.

The river search also involves the river banks and some islands. "They are checking those islands as well," Cooley said. "They are very brushy."

Ground searchers looked through the night but did not find the plane carrying four people: Inter Lake reporters Melissa Weaver and Erika Hoefer, both of Kalispell, and two Missoula men, pilot Sonny Kless and Brian Williams.

Williams is originally from Coeur d'Alene. He graduated from Lake City High School in 1999, according to high school friend Jim Everson.

Shawna Biggerstaff, who also attended high school with Williams, said he visited his hometown often.

"I actually saw him on Saturday night. He was in town and everybody was just hanging out downtown," she said, adding that she thinks he is 29.

He didn't mention anything about leaving on a plane, she said.

"I'm in shock. I can't even believe it," she said. "He was really into mountains and backpacking. That's what he spent his time doing and what he liked to do. Let's hope he's alive and well."

Williams' father Gary Williams, who lives in Coeur d'Alene, could not be reached for comment.

The plane and its occupants had been missing from a Sunday afternoon sightseeing flight.

"We had folks in vehicles and four-wheelers search throughout the night," Cooley said from the search command post at the National Bison Range Visitor Center 80 miles south of Kalispell.

Aerial search efforts ramped up Tuesday morning involving nine planes and two helicopters.

"Our base starting point is that last radar contact," Cooley said. "That's where we started."

Cooley said there have been several eyewitness reports from people who saw the plane flying low just west of the Bison Range.

"There's a consistency in those reports where everybody said the same thing: 'It was flying low,'" Cooley said. "They are using those reports and going out from there."

The Flathead River flows about three miles west of the Bison Range. The river bottom is lush with cottonwoods and brush.

The terrain in the area is mostly open, prairie-grass-covered rolling hills ringed by timber-capped mountains.

The pilot, Kless, last made radio contact with the tower at Glacier Park International Airport at 2:11 p.m. Sunday, reporting that he was east of Kalispell, traveling south to north.

Freebury said cell-phone tower information also was analyzed.

The Associated Press and the Coeur d'Alene Press contributed to this story.