Police plan to take Travis Decker — the Washington man accused of savagely killing his three young daughters — alive when they track him down … despite the military veteran’s dangerous reputation, and the fact he’s possibly armed.

Chelan County Sheriff Mike Morrison tells TMZ it’s unclear if Decker has weapons … but they are tracking him diligently and hope to come into contact with him soon … and their Plan A is to detain him peacefully.

Of course, it’s possible Decker will get aggressive with authorities — and Morrison says any use of force by law enforcement to neutralize a threat “will be determined by what is reasonable and necessary at the time.”

The Wenatchee Police Department issued an endangered missing persons alert on May 31, and the bodies of the three young girls were found near a campground in Chelan County — around 70 miles east of Seattle — on June 2.

Law enforcement has been on a grueling hunt for Decker since … with the search growing so large, Washington state Gov. Bob Ferguson ordered the National Guard to assist authorities in combing parts of central and northern Washington State.

The manhunt’s clearly no walk in the park — Decker is ex-military and is trained in survival skills, with the ability to live in the wildneress for weeks at a time.

There’s also a chance he has plans to or has already crossed over into Canada … as an affidavit obtained by TMZ shows he allegedly searched “how does a person move to canada,” “how to relocate to canada” and “jobs canada.”

The search continues … and Chief Morrison tells TMZ he’s hopeful Decker will turn himself in.