Lt. Roy Butler says nothing suspicious was found. An investigation continues, and police have consulted with the FBI.
The school district says officials are deciding whether classes will resume Thursday.
Butler says the threats were determined to be credible and not a prank.
Multiple written threats were made Tuesday against students and staff at one school in the Vancouver suburb. Four other public schools and one private school nearby also closed as a precaution.
The five schools have a total of about 4,300 students.
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