Tennessee police said today that an investigation into the whereabouts of two students had been sparked by a disturbing tip from one of the teenagers' counselors.

"This wasn't just a threat," Hawkins County Sheriff Ronnie Lawson said today of the allegations. "It was a plan of attack."

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Authorities said that a counselor treating the younger of the two students at a youth center had called police in October. The teen's parent had reportedly discovered massacre plans in a journal and sought help.

The counselor revealed to authorities that the student had shared plans for shooting up his high school during a counseling session, officials said.

"If we have a concern that a youth may be a risk or harm to another individual, we make efforts in the moment to notify the individual who is at risk, as well as to contact authorities," Kandi Shearer of Youth Villages Specialized Crisis Services in Johnson City, Tennessee, said.

Authorities said the teenagers, now 16 and 17, had planned to shoot up Volunteer High School in Church Hill, Tennessee, and kill as many people as possible. The teenagers' names have not been released by authorities because they are juveniles.

During a search of the younger student's home in December, police said, they found several guns, firecrackers, a plastic bag full of gunpowder and floor plans to the high school, as well as journals.

"There was a detailed plan of attack, studies of maximum amount of students in the lunchroom and hallways, where to block the exits," Lawson said.

Police said the younger teen's journals revealed that he also had an obsession with serial killers and the April 1999 killings at Columbine High School in particular, and that he was waiting until his senior year to execute the plan.

They said he'd tried to recruit other students to join him.

Shortly after the counselor made the call, police took both students into custody, where they've remained since the investigation began.

Police charged both Thursday with conspiracy to commit first-degree murder, conspiracy terrorism and possession of explosives. Neither teen had entered a plea.

The school system today said that while students at Volunteer were never in danger, it was wake-up call to students everywhere.