After six days of scouring a remote section of the northern Sierra Nevada, authorities announced Monday they had suspended their search for a missing Santa Rosa couple presumed to have crashed while flying from Truckee to Petaluma.

“They’ve exhausted the search,” said Sgt. Michelle Anderson of the Sierra County Sheriff’s Office, which worked with the Civil Air Patrol, the California National Guard, the California Highway Patrol and other agencies to locate the plane.

Brenda and Mark Richard flew out of the more than mile-high Truckee Tahoe Airport April 17 headed for Petaluma. At the time of their takeoff around 4 p.m., the temperature was 41 degrees with overcast skies and wind gusts exceeding 20 mph.

Pilots departing the Tahoe-Truckee Airport must navigate 8,000- to 9,000-foot peaks shortly after takeoff.

Authorities believe the white-and-blue single-engine Socata TB-20 Trinidad crashed in the snow somewhere in a 400-square-mile stretch of backcountry in Sierra County, 18 miles northwest of Truckee. The rough location of the plane was determined by radar and cellphone data.

The Richards’ plane was equipped with an emergency locater transmitter, said Kathy Johnson of the Civil Air Patrol, a volunteer organization affiliated with the U.S. Air Force. In most circumstances the device would send signals, even if the plane had crashed into a deep ravine, Johnson said, but for an unknown reason the transmitter failed.

With record winter snowfall and nearly the entire search area covered in snow, the white-bottomed plane would be extremely difficult to spot, Johnson said. Plus, there was more snowfall after the presumed crash.

Failing to find a plane is uncommon, Anderson said. Of the five searches for downed aircraft in her 11 years on the force this is the first time a plane has not been located, she said.

The search ended at 9 p.m. Sunday. It had been suspended Saturday due to weather before resuming Sunday morning.

Civil Air Patrol volunteers from California and Nevada flew more than 60 hours over the area while others pored over more than 8,000 aerial digital images, taken by wing-mounted cameras, according to officials.

Brenda and Mark Richard are now presumed missing persons, said Anderson. Temperatures in the mountains have fallen well below freezing most nights since they went missing.

The couple has four daughters as well as a new granddaughter, according to neighbors.

In a statement released Thursday, the Richard family said they were trying to remain positive, but requested privacy. The daughters did not release another statement after the announcement the search for their parents had been suspended.

Staff Writer Randi Rossmann contributed to this report.

You can reach Staff Writer Nick Rahaim at 707-521-5203 or nick.rahaim@pressdemocrat.com. On Twitter @nrahaim.