A Washington state survivalist has been on the run from authorities for nearly a week after allegedly killing his three young daughters, who were found with plastic bags over their heads and bound wrists at a campground in Chelan County Monday.

Travis Caleb Decker, 32, did not have custody of the girls – all of whom are believed to have died from asphyxiation. Decker spent time in the military and is an avid outdoorsman, according to authorities.

“We’re getting more confirmation, that he might have possibly gone through Mountain Survival School,” Chelan County Sheriff Mike Morrison told reporters Wednesday. “But we also know, too, that he’s been an outdoorsman since he was a kid.”

He said the U.S. Marshals and the FBI are assisting with the manhunt.

From left, a composite of undated family photos showing 9-year-old Paityn, 8-year-old Evelyn and 5-year-old Olivia Decker. All three were reported missing Monday after their father failed to return them to their mother’s custody. Now he is wanted in connection with their murders. (Courtesy of the Wenatchee Police Department)

It is unknown if he is armed, but authorities say he should be considered dangerous.

The Chelan County Sheriff’s Office is advising anyone who sees Decker to avoid approaching him and call 911 immediately. He faces a maximum of life in prison without the possibility of parole.

Authorities are offering a reward of up to $20,000 for information leading to his arrest.

“We want a peaceful resolution to this, but we’re not going to relinquish our efforts and give up any effort that we could proceed to track you down,” Morrison told reporters at a news briefing Wednesday. “So please turn yourself in. Let’s wrap this up and do what’s right for your kids.”

Anyone with information is asked to call the Chelan sheriff’s tip line at 509-667-6845 or submit information online at https://www.co.chelan.wa.us/sheriff/forms/submit-a-tip.

Fox News’ Christina Shaw contributed to this report.