Authorities have evacuated about 125 people from the visitors centre at Mount Rainier National Park in Washington, where police have been searching the snowy terrain for an armed gunman suspected of killing a park ranger.

About 150 officers converged on the mountain park after ranger Margaret Anderson was shot to death on Sunday morning. Searchers used an aircraft with heat-sensing capabilities to hunt from the skies.

Authorities believe the gunman was still in the woods, with weapons.

Pierce County sheriff's spokesman Ed Troyer said that Benjamin Colton Barnes, a 24-year-old believed to have survivalist skills, was a "strong person of interest" in the killing.

"We do have a very hot and dangerous situation," Troyer said.

Teams were assessing new tactical plans that they planned to put into place at daylight, he said.

Safety concerns prompted authorities to keep the tourists quarantined at a visitors center as the manhunt unfolded. Early on Monday morning, officers escorted them out of the park in groups of vehicles over several hours.

Crews had initially planned to keep everyone in a basement with guards. But it was determined to be "better to do it (evacuate) under the cover of darkness than daylight," Troyer said.

Evacuee Dinh Jackson, a mother from Olympia who came to the mountain to sled with family and friends, said officials ordered people to hurry into the lodge after the shooting.

Jackson said officials had everyone get on their knees and place hands behind their heads as they went through the building, looking at faces to make sure the gunman was not among them.

"That was scary for the kids," she said.

Michael Wall, an elementary school teacher from Puyallup, spent the morning hiking with his son. They didn't find out about the violence until returning.

Wall said he was impressed by how staff members and visitors kept each other comfortable with food and conversation.

"It was calm, cool, easygoing," Wall said. "I didn't notice any tenseness or terseness."

A parks spokesman said Barnes was an Iraq war veteran, and the mother of his child had alleged he suffered from post-traumatic stress following his deployments.

Barnes was involved in a custody dispute in Tacoma in July, during which the toddler's mother sought a temporary restraining order against him, according to court documents.

In an affidavit, the woman wrote that Barnes was suicidal and possibly suffered from PTSD after deploying to Iraq from 2007 to 2008. She said he gets easily irritated, angry and depressed and keeps an arsenal of weapons in his home.

Barnes was also a suspect in the shooting of four people at a house party south of Seattle, police said.

Sergeant Cindi West, King County sheriff's spokeswoman, said Barnes was connected to an early-morning shooting at a New Year's house party in Skyway, south of Seattle that left four people injured, two critically.

That shooting happened about 3am and stemmed from an argument over a gun.

West said three people fled the scene. Two were located, and West said authorities were trying to find Barnes and had been in contact with his family, trying to have them convince him to "come to the police and tell his side of the story" in the Skyway shooting.

At Mount Rainier around 10.20am. Sunday, the gunman had sped past a checkpoint to make sure vehicles have tire chains, which are sometimes necessary in snowy conditions, Bacher said.

One ranger began following him while Anderson, a 34-year-old mother of two young children who was married to another Mount Rainier park ranger, eventually blocked the road to stop the driver.

Before fleeing, the gunman fired shots at both Anderson and the ranger that trailed him, but only Anderson was hit, Bacher said. Anderson would have been armed, as she was one of the rangers tasked with law enforcement, Bacher said. Troyer said she was shot before she had even exited the vehicle.

About 150 officers, including officials from the Washington state patrol, US Forest Service and FBI, were on the mountain Sunday.

Tactical responders wearing crampons and snowshoes pursued what appeared to be the gunman's tracks in the snow, Troyer said. Those tracks went into creeks and other waterways, making it more difficult for crews to follow.

"He's intentionally trying to get out of the snow," Troyer said.

Authorities recovered his vehicle, which had weapons and body armoir inside, Troyer said.

A Swat team was able to remove Anderson's body from the mountain late Sunday night, with a procession of law enforcement vehicles escorting her remains away.

The park would remain closed Monday, officials announced.

Park superintendent Randy King said Anderson had served as a park ranger for about four years. King said Anderson's husband also was working as a ranger elsewhere in the park at the time of the shooting.

"It's just a huge tragedy - for the family, the park and the park service," he said.

Adam Norton, a neighbor of Anderson's in the small town of Eatonville, said the ranger's family moved in about a year ago. He said they were not around much, but when they were, Norton would see Anderson outside with her girls.

"They just seemed like the perfect family," he said.

The town of about 3,000 residents, which is a logging community overlooking Mount Rainier, is very close knit, he said.

"It's really sad right now," Norton said. "We take care of each other."

It has been legal for people to take loaded firearms into Mount Rainier since 2010, when a federal law went into effect that made possession of firearms in national parks subject to state gun laws.

Bill Wade, the outgoing chair of the Coalition of National Park Service Retirees, said Congress should be regretting its decision to allow loaded weapons in national parks. He called Sunday's fatal shooting a tragedy that could have been prevented. He hopes Congress will reconsider the law that took effect in early 2010, but doubts that will happen in today's political climate.

The shooting occurred on an unseasonably sunny and mild day. The park, which offers miles of wooded trails and spectacular vistas from which to see 14,410-foot Mount Rainier, draws between 1.5 million and 2 million visitors each year.

The Longmire station served as headquarters when the national park was established in 1899. Park headquarters have moved but the site still contains a museum, a hotel, restaurant and gift shop, which are open year-round.