FAIRFAX, Va. (AP) — An 18-year-old has been arrested and charged with making threats to attack a northern Virginia high school last month.

Police on Monday said Ishmael Harrison turned himself in on charges of threatening to harm people on school grounds. He was also charged with possession of child pornography.

A search of Harrison’s home, authorities said, yielded over 200 hundred rounds of ammunition. The type, they said, is commonly used in an AR-15.

Harrison allegedly posted threats on social media Feb. 21 targeting students and staff at Fairfax High School, as well as police.

“These threats were widely distributed using social media networks, and they created a tremendous amount of fear and anxiety within the community at large,” said Carl Pardiny, Fairfax City police chief, during a news conference Monday afternoon.

“While threats of the nature are occurring all across our nation, both the city of Fairfax and Fairfax County police departments are absolutely committed to the safety and welfare of our communities,” he said, “and we take all threats seriously.”

Authorities credited students for speaking out when they saw the treats online.

“As soon as [students] saw these threats on Twitter, that’s when they started notifying parents, teachers, school staff and the Police Department,” said Chief Edwin Roessler of the Fairfax County Police Department.

Fairfax City police had issued a press release Saturday saying they were searching for Harrison, a senior at Fairfax High, and considered him armed and dangerous. Their concern increased when they executed a search warrant at the student’s home Friday and found the rifle ammunition but could not find Harrison himself.

WTOP’s Kristi King and Jack Pointer contributed to this report.