A military veteran suspected of killing his three daughters could pose a “significant risk”, US authorities have warned as they search for the wanted man.

Travis Decker, 32, from Washington state, is wanted on first-degree murder charges over the killings of Olivia, Evelyn and Paityn Decker, Wenatchee police said on Tuesday.

The girls, ages 5, 8 and 9, were found dead on Monday after Decker’s truck was discovered near a campground about 210km east of Seattle, police said.

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Decker was not with the truck. His daughters’ bodies were found during a search of the immediate area.

Additional details about their deaths or a possible motive were not immediately available.

Authorities began searching for the girls after their mother filed a civil complaint on Friday night when Decker did not bring them home after a planned visitation, the department said.

The department sought an amber alert from state police that night, according to the release, but its request “did not meet the required criteria”.

Washington State Patrol said the request did not meet two criteria of the alert system, a federal program with strict guidelines set by the US Department of Justice: there must be a reason to believe the missing person has been abducted and there must be a known, imminent danger of serious injury or death.

Decker had some custodial rights, so abduction was not an immediate legal presumption, spokesperson Chris Loftis said.

“In a situation where a custodial parent is late returning children to another parent, law enforcement does not automatically have clear indication that the children are in danger,” he added.

On Saturday, the state patrol issued an endangered missing person alert, meant for people who are unable to “assist in their own recovery,” Loftis said.

That alert, he said, “accomplished much the same public notification/outreach as an amber alert would have”.

Decker’s truck had been seen traveling west from Wenatchee, police said, and authorities continued searching for the girls over the weekend.

With the assistance of the FBI, law enforcement officers narrowed their focus to an area near Leavenworth, roughly 27km east of the area where Decker’s truck was found the next day at 3.45pm, police said.

Authorities initially obtained a warrant on three counts of custodial interference against Decker.

He is now wanted on three counts of first-degree murder and first-degree kidnapping, the police department said.

It described Decker as a military veteran with extensive training. It isn’t clear whether he is armed.

Because of safety concerns, police warned anybody who sees Decker not to contact or approach him.