Two men set up a rifle with a telescopic lens before firing on two Australian firefighters and two US Forest Service personnel patrolling in remote Washington state near the White Pass ski area, according to staggering new details contained in a debrief note prepared about the incident.

Written by US Forest Service officers, the note shows the incident began at around 1.30pm on August 23, when the first shots were heard, and only ended at 6pm after the firefighters had been rescued.

NSW firefighters Daniel Barwick, left and Phillip Eberle, right, are the two Australians identified as being involved in the incident.

"The incident personnel watched with binoculars as one of the two individuals put a scoped rifle on a bi-pod and looked up the mountain toward them," it reads.

"They continued to wave until a shot was fired ... they then bailed off the backside of the mountain to take cover as a second shot rang out."