A popular 24-year-old math teacher at Danvers High School in Massachusetts, Colleen Ritzer, has been found dead in woods near the school, and a 14-year-old boy has been arrested in connection with her death, officials said Wednesday.

“It is apparent that she is a homicide victim,”  Jonathan Blodgett, the district attorney in Essex County, Massachusetts said at a news conference. “This is a terrible tragedy for the family of Colleen Ritzer and the entire Danvers family.”

All Danvers schools have been closed for the day. Authorities said the high school remained an active crime scene.

Ritzer's death came after a Nevada middle school teacher was shot and killed Monday when a student opened fire on campus, wounding two other students, before turning the gun on himself.

Blodgett said the Danvers investigation began when a missing-person report was filed Tuesday on a 14-year-old boy who had not returned home from Danvers High that day. Police learned Tuesday night that Ritzer also had not returned home and was not answering her cellphone.

When police searched for her, they discovered blood in the second-floor bathroom of the high school, and began to search school property.

Early Wednesday morning, police responded to a report of a pedestrian walking on a busy road, who turned out to be the missing boy.

The boy will be arraigned Wednesday on a murder charge, Blodgett said. The proceeding will be closed to the public because the suspect is a juvenile.

Ritzer's death is being mourned in Danvers and online.

On a Twitter account for a Colleen Ritzer, the account profile identifies her as a math teacher in Danvers who is “often too excited about the topics I’m teaching.”

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alana.semuels@latimes.com

Twitter: @AlanaSemuels