A Maryland couple alerted authorities of their daughter's plan to launch a violent attack at her high school, police revealed Monday (27 March).

The 18-year-old student is in police custody after being evaluated at a hospital for being a threat to herself and to those at the school.

Nichole Cevario was charged with possession of explosive material with intent to create a destructive device and possession of incendiary material with the intent to create a destructive device, the Frederick County Sheriff's Office said.

Investigators said in a statement that they deemed the threat as serious, given that Cevario was "actively acquiring materials needed to execute" violence at Catoctin High School in Thurmont, the Washington Post reported.

Police found a shotgun, ammunition and bomb-making materials, including pipes, shrapnel, fuse material and fireworks at Cevario's home. The Baltimore Sun reported that the federal bureau of Alcohol, Tobacco, Firearms and Explosives, along with the Office of the State Fire Marshal, determined that the materials had not been combined to create an explosive. All the items were purchased legally.

The teenager outlined her plans to carry out a shooting at her high school in a journal, scheduling 5 April as the attack date, investigators said.

Authorities said the journal showed Cevario had been "planning this event for some time and compiling intelligence on behaviour activities". Among her notes were the school's emergency procedures and "intelligence" on security personnel assigned to the school.

Authorities added that the journal included a timeline of "how she was going to execute the plot" and her "expectations at each stage of the event," according to the Post. "It was very chilling to read it," Frederick County Sheriff Charles Jenkins said. "We've never seen anything like this in this county, and I go back 27 years."

Major Tim Clarke, a sheriff's spokesman, said the teen's parents reported their concerns to the school on Thursday (23 March) and that authorities became involved. Cevario was taken from class and transported to a hospital for an emergency evaluation. Officials said that the teen had not taken weapons or explosive devices to the school.

Thwarted Maryland HS attack
Materials recovered by Frederick County Sheriff's Office in a plot to launch a mass shooting at a Maryland high school Frederick County Sheriff's Office

"This potentially could have been a serious incident," Clarke said. He added that officials are "confident that the safety" of the school is "fine based on an investigation."

At a news conference on Monday, Jenkins said he was "shocked" by the incident. "It hits home. I actually live in Thurmont, and it's home to a lot of us," the sheriff said. "But just the fact that a young woman had this in her mind, and this way of thinking and planning out and carrying out an attack much like Columbine or Sandy Hook...it shocks the conscience to see that someone that age could be thinking like this."

According to The Baltimore Sun, police believe the Cevario conceived of the plot on her own and planned to carry out alone.

Jenkins said the teen's parents have been cooperative in the investigation. "I want to say again, I'm extremely thankful for her parents for their actions bringing this information forward," he said. "I truly believe what they did thwarted a catastrophe at Catoctin High School."