Authorities say they believe they spotted Travis Decker, an ex-soldier wanted in the deaths of his three daughters, near a remote alpine lake in the US state of Washington, after receiving a tip from hikers who said they saw a lone person who appeared to be ill-prepared for the conditions.

The Chelan County Sheriff’s office said in a Facebook post that tracking teams responded immediately, and a helicopter crew spotted a hiker near Colchuk Lake, in a popular Cascade Range backpacking area called The Enchantments.

The off-trail hiker ran from sight as the helicopter passed, the sheriff’s office said.

Teams later found a trail, and K-9 teams tracked the person to the area of the Ingalls Creek Trailhead, south of Leavenworth.

Authorities did not say when they spotted the subject, but late on Monday night they issued an alert for residents in the Ingalls Creek and the Valleyhi community to lock homes and vehicles and to be on the lookout for Decker.

Decker, 32, has been the target of a large manhunt ever since June 2, when a sheriff’s deputy found his truck and the bodies of his three daughters — nine-year-old Paityn Decker, eight-year-old Evelyn Decker and five-year-old Olivia Decker — at a campground outside Leavenworth.

He had failed to return the girls to their mother’s home in Wenatchee, about 160km east of Seattle, following a scheduled visit three days earlier.

Decker was an infantryman in the US Army from March 2013 to July 2021 and deployed to Afghanistan for four months in 2014.

He has training in navigation, survival and other skills, authorities said.

He once spent more than two months living in the backwoods off the grid.

Officials with an array of state and federal agencies have searched hundreds of square miles, much of it mountainous and remote, by land, water and air.

The US Marshals Service was offering a reward of up to $US20,000 ($30,000) for information leading to his capture.

Last September, his ex-wife, Whitney Decker, wrote in a petition to modify their parenting plan that his mental health issues had worsened and that he had become increasingly unstable, often living out of his truck.

She sought to restrict him from having overnight visits with their daughters until he found housing.

An autopsy on Friday determined the cause of death to be suffocation, the sheriff’s office said.

The girls had been bound with zip ties and had plastic bags placed over their heads.