The man accused of opening fire at a Florida high school on Wednesday, killing 17 people, was a troubled former student who loved guns and was expelled for unspecified disciplinary reasons, police and former classmates said.

Nikolas Cruz, 19, was arrested about an hour after a shooting rampage at Marjory Stoneman Douglas High School in Parkland, Florida, Broward County Sheriff Scott Israel told reporters.

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Cruz, who had been expelled from the school for reasons that have not been made public, was found with multiple ammunition magazines and one AR-15-style rifle, Israel said.

"We already began to dissect his websites and the things on social media that he was on and some of the things that came to mind are very, very disturbing," Israel said.

Chad Williams, 18, a senior at Stoneman Douglas High school, remembered Cruz as a troubled classmate from middle school. He said Cruz would set off the fire alarm, day after day, and finally got expelled in the eighth grade.

More recently, Williams saw Cruz carrying several publications about guns when they ran into each other at the high school. Williams thought Cruz was there to pick up a younger sibling.

"He was crazy about guns," Williams told Reuters by the side of the road near the high school. "He was kind of an outcast. He didn't have many friends. He would do anything crazy for a laugh, but he was trouble."

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Jillian Davis, 19, said she was in a school Junior Reserve Officer Training Corps with Cruz in the 9th grade. She remembered him as a quiet and shy young man who would almost change personality when angry. He talked a lot about guns and knives but no one took him seriously, she told Reuters.

"I would say he was not the most normal or sane kid in JROTC. He definitely had a little something off about him. He was a little extra quirky," said Davis, who graduated from the school last year.