The search for Lindsey Baum, a 10-year-old girl who vanished from a small town in Washington nine years ago, ended this week when officials announced that her remains had been identified.

They are now focused on finding her killer.

Lindsey’s mysterious disappearance, a missing persons case that made national headlines, has become a kidnapping and homicide investigation, Sheriff Rick Scott of Grays Harbor County said at a news conference on Thursday, adding that he would remain involved until the “monster” who killed the girl was held accountable.

“We’ve brought Lindsey home. We’ve recovered her,” Mr. Scott said. “Sadly, she was not recovered as we and her family had hoped and prayed these last nine years.”

The F.B.I.’s Seattle field office and the sheriff’s office are asking for the public’s help as they try to track down the person or people responsible for her death. The F.B.I. referred all questions about the case to the sheriff’s office, which is not staffed on the weekend and did not immediately respond to a request for additional information.