3 sisters allegedly killed in Washington during visit with their father were suffocated: Authorities

Three young sisters who were found dead near a Washington campground after they left home for a "planned visitation" with their father died from suffocation, authorities said Monday.

Paityn Decker, 9; Evelyn Decker, 8; and Olivia Decker, 5, were located on June 2 near the Rock Island Campground in Chelan County, Washington, following a search, police said.

An autopsy completed on Friday determined the girls' cause of death to be suffocation and the manner of death is homicide, according to the Chelan County Sheriff's Office.

The girls had each been found with plastic bags over their heads and their wrists were zip-tied, according to court documents previously obtained by ABC News.

Their father, Travis Decker, who is wanted for their murders, remains at large.

His truck was recovered at the scene where his daughters were found, authorities said.

The Chelan County Sheriff's Office said Monday it has "collected a large amount of evidence, many of the suspect’s personal items," from the truck, though the analysis of the evidence "creates more questions and information for investigators to continue to pursue."

Blood samples taken from the scene included human and non-human, the sheriff's office said. Authorities had previously said in court documents that the tailgate of the truck had what appeared to be "two hand prints of blood."

"Further DNA and fingerprint analyses are still being conducted," the sheriff's office, which is leading the criminal investigation, said.

The suspect's dog has also been recovered and turned over to a humane society for care, the sheriff's office said.

Federal authorities are now leading the manhunt for Decker, with hundreds of square miles searched so far.

Law enforcement members have searched the forested areas and dozens of structures that Decker was last known to be in, according to the Chelan County Sheriff's Office.

Decker, who is homeless and has been living in his vehicle or at various motels or campgrounds in the area, is currently wanted for three counts of first-degree murder and three counts of first-degree kidnapping, police said. He is an Army veteran with "extensive training," but it is unknown whether he is currently armed, officials said. Decker was deployed to Afghanistan in 2014, according to a spokesperson for the Washington State Military Department.

Arianna Cozart, the attorney for Whitney Decker, the ex-wife of the suspected murderer and mother of the three girls killed, told ABC News last week that Decker struggled with mental health issues, including PTSD, and was unable to receive help through veterans' resources.

"The courts didn't fail these girls. It wasn't the judge and it wasn't Whitney; it was our system," Cozart said.

The father "had some mental health issues and some instability in his life" that led to restrictions regarding overnight visitation and taking his daughters out of the area, but before the girls disappeared, "there were no red flags," Cozart said.

Decker, who is described as 5 feet, 8 inches tall with black hair and brown eyes, was last seen wearing a light shirt and dark shorts, according to police.

Officials said anyone who has any information on Decker or knows of his whereabouts should call 911 immediately.

ABC News' Megan Forrester contributed to this report.