Authorities disclosed the alleged plot on Wednesday, saying the boy planned a June attack at Belvidere High School, in a small Victorian town 60 miles west of Manhattan.
The 17-year-old high school senior has not been arrested or charged with a crime. State Police spokesman Lt. Gerald Lewis would not say on Thursday whether he would be charged. He has been involuntarily committed to a hospital for psychiatric evaluation.
Officials are not releasing his name because he is a juvenile.
One man has been charged in the case. James M. Shipps, 22, is accused of helping to plan the attack with the teen. He has been charged with creating a false public alarm, making terroristic threats and hindering apprehension in the case.
Sweigert said the alleged plotters planned to use weapons from the 17-year-old's home and to order some additional supplies.
Shipps is being held on $50,000 bail. By Thursday afternoon, it was unclear whether he had a defense lawyer.
Sweigert said the teen had been in trouble with police in the past, but was generally a decent student with plans to attend community college after he graduated.
But in the last few weeks, he changed drastically, police said.
Sweigert said the teen became obsessed with going to war and started dressing like a soldier, even carrying an Army manual around school. He talked of joining the New Jersey National Guard like Shipps, who was said to have told people that he was going to Iraq soon - though the Guard deployment orders for later this year had not gone out yet.
The teen, authorities said, was rejected recently by the New Jersey National Guard because he was on probation for an incident last year in which he was caught firing hollow-point bullets from a handgun in the woods.
It was the teenager's more recent behavior, however, that caused officials in the school to call police on Sunday.
Monday morning, school and police officials spoke with the teen and were troubled by what he told them.
"He told me himself: 'I love combat and I want to kill,"' Sweigert said.
With permission from the boy's father, who also let police take the guns from his home, arrangements were made to take him in a police car from school to a hospital.
After the boy was removed from school, investigators dug further, and found disturbing journals, but no specific plans to attack, according to Dirk Swaneveld, the superintendent of schools in Belvidere.
"There was no list, there was no written organized plan," Swaneveld said.
Like in other cases where attacks have been threatened but not carried out, it's impossible to know whether a tragedy was truly imminent. But school officials in Belvidere believe the teen's behavior was troubling enough to justify action.
"We still felt the threat to be real," Swaneveld said, and so did police.
The superintendent said he spent part of Thursday trying to calm parents upset that they hadn't been told of the prospect of the attack before late Wednesday afternoon.
Security at the school was increased for a few days, and Chief Sweigert was considering what measures would be needed when the 17-year-old is released from the hospital.
"He's still a threat," Sweigert said. "We don't know what's going to happen."
