She was last seen at 9 p.m. the night before in her bedroom.
Authorities don't believe she ran away or left voluntarily with someone.
"We believe that, more likely than not, this is a legitimate abduction," said Chief Criminal Deputy Dean Byrd.
There are no suspects.
Deputies have canvassed and re-canvassed the neighborhood near the town of Shelton, about 45 miles southwest of Seattle. They have no leads, Byrd said.
By Wednesday afternoon, a Washington State Patrol trooper and FBI agent were also on the scene.
The federal agency is assisting the sheriff's office, FBI spokeswoman Ayn Dietrich in Seattle confirmed.
The sheriff's office also has asked for crime scene investigation help from the Washington State Patrol crime lab, Byrd said.
Gaspar-Guerrero is Hispanic, 5-foot-2 and weighs 128 pounds. She has brown eyes and black hair just past her shoulders. She is believed to be wearing black shorts and a black blouse.
The family received a call Wednesday from an unidentified person who said the girl was found, but authorities believe that was a false report, Byrd said.
The parents didn't know who it was from, and there was no such call to dispatchers.
"At this point, it's unsubstantiated if not malicious," Byrd said.