The school was placed on modified lockdown after receiving the threat. The school was later evacuated and its 2,000 students were sent home at about 10:30 a.m. as part of a voluntary early dismissal.
Sgt. Cindi West of the King County Sheriff's Office said the emailed threat came in early Friday morning.
"I understand this email said something about being prepared for a hostage takeover, and it will be a bloodbath," she said. "Kentwood High School immediately notified the King County Sheriff's Office."
K-9 police dogs were being brought in to search the school, but nothing was found, West said.
One student, Stephanie Ogden, told KOMO Newsradio that school officials never explained why they were being sent home.
"They just told us to get off campus as quickly as possible," she said.
West said it appears the emailed threat came from a former student who sent the message after hacking into another student's account.
Sheriff's investigators are working with the Kent police on locating the suspect.
"This is a serious matter," said West. "We take these things seriously. This is a crime ... it's not funny, and this person will likely be charged with making threats."
She said the Sheriff's Office would not say flat out that the threat was a hoax, but rather rather "the KCSO investigated and nothing was found."