Grim details revealed in the deaths of three Washington girls as the search for their father continues

Three girls believed to have been killed by their father were found zip-tied and with plastic bags over their heads in a remote part of Washington state, according to court documents obtained Wednesday.

Three girls believed to have been killed by their father were found zip-tied and with plastic bags over their heads in a remote part of Washington state, according to court documents obtained Wednesday.

A preliminary examination concluded that the girls, ages 5, 8 and 9, likely died by asphyxiation, according to an affidavit in support of an arrest warrant for their father, Travis Decker.

A full autopsy is expected to be released Thursday, Chelan County Sheriff Mike Morrison said at a Wednesday evening news conference, noting that the girls are a "motivating force for us" in the search for Decker.

Decker, 32, is wanted on charges of first-degree murder and kidnapping. The U.S. Marshals Service on Wednesday announced a $20,000 reward for information leading to his capture.

The military veteran is not known to be armed but should be considered dangerous, officials said.