A 56-year-old female teacher was shot dead on Monday by a teenage male pupil in a high school in Estonia, the first case of its kind in the Baltic state.
The 15-year-old assailant, who was taken into custody, used a gun legally registered to his father, police said.
"The shooting took place during a German lesson and the teacher was fatally injured," police spokeswoman Kristina Kostina told AFP.
"There are no other victims or injured," she added.
The school principal said the pupil, whom he described as an average teen, remained very calm after placing the revolver on a table following the killing.
President Toomas Hendrik Ilves called the unprecedented school shooting "a tragedy for all of Estonia". He offered his condolences to the family of the dead teacher.
"This is not just a tragedy of one school or one town.... This is grief and a shock for all of us," Ilves said in a statement.
Estonia's education minister went to the crime scene in the central town of Viljandi.
The motive for the attack remained unclear.
However Estonia's leading Postimees newspaper reported that the unidentified boy had made disturbing posts on social media including Facebook alongside photographs of guns and war.
"Don't judge me because I'm quiet. No one plans a murder out loud," Postimees quoted him as saying in an April 1 post.
The Postimees daily also reported the killer had made disturbing comments on Ask.fm, a popular question and answer social networking site.
Yet another post was prompted by the question "If you became a multi-millionaire overnight, what would you buy?"
To which the teen replied: "An army", according to Postimees.
Estonian media quoted the high school principal as describing the boy as a "very well-behaved young man" who previously had no trouble at school.
"His achievements in school were not bad; an average pupil," principal Aavo Palo was quoted as saying by Estonian media.
"I don't have any information that the teacher... had any conflicts with students previously and up until now I have no idea what exactly happened in this case," Palo said.
The teen faces up to 10 years in prison if convicted of murder as a minor.