The 15-year-old boy who shot himself after taking 23 students and a teacher hostage at his school in Wisconsin has died of his wounds.
Marinette Police Chief Jeff Skorik said Samuel Hengel died on Tuesday morning at a hospital in Green Bay.
Hengel shot himself with a handgun as police stormed a classroom at Marinette High School on Monday night, after a stand-off lasting several hours.
Chief Skorik said police were still trying to ascertain his motives.
"As far as what caused this, it seems to be a mystery," he said. "We have not been able to identify anything that precipitated this incident."
'Most respectful kid'
He said the incident at Marinette High School began at about 1330 (1930 GMT) on Monday, sometime after which the first shots were fired.
One of the hostages said Hengel left the classroom while a film was being shown and came back with a duffel bag, which police later said contained two semi-automatic handguns and ammunition.
He then reportedly shot at the film projector and a wall, before sitting down at the front of the class.
"He didn't say anything," Austin Biehl, 15, told reporters. "We were just scared and shocked he was doing this. My legs were shaking."
The authorities were first notified of the incident by the high school's principal at 1548, after he was told of the situation by a student whom Hengel had allowed to leave.
After a stand-off of four hours - during which the teacher, Valerie Burd, communicated with the hostage negotiation team and Hengel's classmates tried to keep him calm by chatting with him about hunting, fishing and movies - five students were released because they needed to go to the toilet.
Chief Skorik said that at 2003, officers outside the classroom heard three shots and "breached the door". At the front of the room was Hengel, who then shot himself in the head, he added. The remaining hostages were evacuated.
Hengel was transported to the nearby Bay Area Medical Center, and later to St Vincent Hospital in Green Bay, where he died.
Family friends of the Hengel family said he was "the most respectful kid you would ever meet" and that his actions were completely shocking.
"I don't think he intended to hurt anyone," Austin Biehl said. "If he wanted to, he probably would have done it."
Classes at Marinette High School will resume on Wednesday.