CHESTERFIELD — A South Carolina teen accused of plotting to bomb his high school is a straight-A student whose parents sought help from mental health experts when he slammed his head into a wall last week, authorities said Monday.

But Ryan Schallenberger, a quiet 18-year-old with a slight build, apparently had been planning his potentially deadly scheme for months.

"This kid had the intellect and the means and the materials to carry it out," county prosecutor Jay Hodge said.

Schallenberger was arrested Saturday after his parents called police because the teen had ordered 10 pounds of ammonium nitrate, an explosive commonly used as fertilizer, from a Web site. Police also said they discovered a hate-filled journal lauding the Columbine killers, an audiotape to be played after Schallenberger perished during his rampage and a year's worth of plans for the bombing that included a hand-drawn map of the school.

"He's just a soft-spoken little kid," Hodge said. "But he's done something and threatened to do something that's very violent."

Schallenberger was charged with making a bomb threat and will be charged Tuesday with possession of bomb-making materials, Hodge said. He was assigned a lawyer, William Spencer, who did not return calls from The Associated Press.

During a brief court hearing Monday, the teen was silent but appeared agitated — his eyes widening at the sight of cameras awaiting him when he entered in an orange jumpsuit, hands cuffed and ankles shackled.

His mother and stepfather, John and Laurie Sittley, could not be reached for comment. Their phone number was unlisted, and they did not attend the court hearing. Passage to their home on a dirt road about 10 miles from the school was blocked by "No Trespassing" signs, locked gates and metal bars stuck into the ground. Authorities said Schallenberger has eight siblings and step-siblings; they were not sure how many live at the home.

Authorities said Schallenberger's journal did not specify targets of an attack or a date that he planned to carry it out. Randall Lear, the town's police chief, said Schallenberger was "just mad at the world" and could have killed dozens, depending on where the teen placed explosives and whether he added shrapnel to them.

Hodge said the teen was set to graduate this year and had no history of bad behavior. Schallenberger was on the school's academic bowl squad and had won an award from a local college a year earlier.

Hodge said he plans to ask that Schallenberger undergo a mental evaluation when the teen appears in court Tuesday for a bail hearing. The teen's parents took him to a hospital three days before his arrest after he smashed his head into a wall in their home, making a 4-inch indentation in the wallboard, Hodge said.

Schallenberger was not badly injured, but his parents also called a local mental health clinic that offered no help, the prosecutor said. A spokesman for the clinic would not confirm or deny any contact with the family, citing state law.

Sheriff Sam Parker said the teen's "heartbroken" parents deserved praise for calling authorities when they retrieved the ammonium nitrate from a post office after receiving a delivery notice at home. The teen was arrested as he walked home from a relative's house.

"Without the parents, Chesterfield County would've suffered. We thank them," Parker said.

Students said the teen often ate alone in the cafeteria and that they were surprised at the accusation. "I never though he'd be the dude to do something like this," said James Ford, a 16-year-old sophomore.

Some adults who had met the teen said the accusations were surprising.

"He wouldn't hurt a flea," said neighbor Carl Parker, who described Schallenberger as well-mannered and a sports fan who used to watch a neighbor's pets when he was younger. "People just don't know him like I do. He's a good kid."

John Davis, a writer for the Progressive Journal, said Schallenberger shadowed him last fall to learn about journalism. Davis said Schallenberger did not do anything that day that hinted at the bombing scheme.

He was "shy, a little bit nervous, kind of soft-spoken. Very intelligent," Davis said. "He caught on quickly."

Authorities checked the school for bombs over the weekend, and on Monday students walked through newly installed metal detectors and past law officers. Still, officials said 60 percent of the school's 544 students stayed home.

Tim Pollard said he kept his daughter out of school as a precaution. "I feel very confident in our police and everything they've done to make sure everything is safe," Pollard said. "It's just one of those deals — better safe than sorry."

Parker, the sheriff, said Schallenberger bought the ammonium nitrate off eBay. A spokeswoman for the Internet auction site said the substance is legal to sell though it must be specially shipped.