One is a 15-year-old who was recently suspended from his suburban high school for making violent threats, according to police. The other is an awkward 17-year-old hungry for attention, acquaintances say.

Together, they envisioned a bloody assault on students and staffers at a Long Island high school, chillingly planning it for the ninth anniversary of the deadly Columbine High School rampage, Suffolk County police said.

“I will start a chain of terrorism in the world,” the 15-year-old wrote in one of several alarming journal entries, according to police. “This will go down in history. Take out everyone there. Perfecto.”

The two students were arrested Friday in the alleged plot at Connetquot High School in Bohemia, about 50 miles east of New York City. Both were charged with misdemeanor conspiracy, punishable by up to a year in jail.

Local students and parents shuddered at the description of the plot, which police said was fleshed out in a videotape that identified several potential victims by name.

“They’re not really afraid to do anything,” a Connetquot student who said he knew the suspects, Joseph Welischar, said. “So, yeah, it’s kind of scary.”

It was more than scary to a mother of two Connetquot students, Dawn Wiegard.

“Its devastating,” she said.

The 15-year-old, a Connetquot student whom authorities identified as the driving force behind the plan, did not enter a plea when he appeared in juvenile court Friday. A judge ordered two weeks of medical evaluation at the Sagamore Children’s Hospital, according to Newsday. The teenager’s mother and stepfather were in the courtroom but would not comment.

A former friend who said she believed she was one of the 15-year-old’s targets told Newsday the suspect was an emotionally troubled boy riveted by violence, and particularly by the 1999 Columbine massacre in Littleton, Colo., in which 12 students and a teacher were killed.

“I do remember him being fascinated about blowing up things,” said Briana Valentino, 15, of Ronkonkoma, who said she and the suspect had been close friends before he suddenly became hostile to her a month ago. “I remember him showing me videos of crazy shootings online.”

The other suspect, 17-year-old Michael McDonough, pleaded not guilty at his arraignment Friday. His Legal Aid Society lawyer, Robert Flick, said his client was “barely” culpable in the alleged plot, according to news reports.

McDonough’s father told the court that his son, who attended Sachem North High School, was receiving mental health counseling.

McDonough’s bail was set at $25,000 cash or $50,000 bond. He was still being held early Saturday, according to Suffolk County jail records.

A shy teenager who wanted to be noticed, McDonough needled fellow students, teenagers who know him said.

“He’s always doing stuff in class to get people to look at him,” Welischar said. “He makes fun of everyone just to get people (annoyed). He doesn’t really like being around people.”

The two suspects were co-workers at a McDonald’s. Police said they targeted scores of students in an attack they planned for April 20, 2008 — the anniversary of the Columbine shootings. The two students responsible for that attack took their own lives.

On July 6, school authorities obtained the 15-year-old’s handwritten journal, which contained “numerous terrorist threats and plans to attack the school,” police said. The journal was turned over to the school after the teenager apparently dropped it in the McDonald’s parking lot, Suffolk County Police Commissioner Richard Dormer said.

“He felt that everyone was against him,” Mr. Dormer said of the 15-year-old, whose name was withheld due to his age. “He was upset at life in general and the world in general.”

Authorities also recovered videotapes made by the 15-year-old — videos Mr. Dormer described as “akin to the tapes that we all saw from Columbine.”

A search warrant was issued for the 15-year-old’s computer. Mr. Dormer said the teenager tried several times to buy five pounds of explosive black powder and an Uzi. The police commissioner said authorities were investigating to see whether any weapons might have been acquired online.

More than 2,000 students attend Connetquot, which is in a working-class community near Brookhaven on the eastern end of Long Island. The school building is open during the summer but does not hold classes.