RIVERHEAD, N.Y. - A teenager plotted with another to pull off a Columbine-style attack on his former high school, authorities on Long Island said yesterday - the second time in three years he has been accused of such a plot.

Christopher Franko, who turned 18 yesterday, was ordered held without bail after his court-appointed attorney entered a not-guilty plea to felony conspiracy. Franko and Dana Saltzman, 16, were arrested in early May on misdemeanor conspiracy, but a grand jury voted to upgrade the charges.

The teens were plotting to attack Connetquot High School tomorrow with shotguns and explosives, prosecutors said. Their plan was to kill security guards outside of the school, then go inside and shoot as many students as possible, Assistant District Attorney Glenn Kurtzrock said. Once police arrived at the school, the pair planned to fire at officers before ultimately killing themselves, the prosecutor added.

Their plot was thwarted in part when their attempts to buy a shotgun at a local sporting-goods store were rejected because neither teen was 18. Prosecutors say Franko intended to return to the store today to make the purchase.

Kurtzrock said he could not explain a motive, other than Franko had apparently been teased by classmates - a slight he never forgot.

"He's definitely a danger," Kurtzrock said, adding that the case is "chillingly similar" to Franko's arrest in 2007 as a juvenile on charges of plotting to attack the school.

That case was handled in Family Court; Franko left Connetquot High School and graduated from a special-education program where he met Saltzman, who is still a student.

She is expected to be arraigned June 17 on the same felony conspiracy count. Her attorney, Cesar Galarza, said she intended to plead not guilty. "This was all his doing," Galarza said. "He basically tried to take a 16-year-old and get her to carry out his plan, which she never intended to carry out."

The prosecutor noted that had Franko remained at the school, he would have been a graduating senior this month. "This was really his last opportunity to get all of these kids in the same place at the same time," Kurtzrock said.