Three teenagers accused of planning shootings at a Mandeville high school that opens for classes Monday intended to target a specific student and a faculty member, St. Tammany Parish Sheriff Rodney Strain said Friday.

The students, who had formed a group called “Day Zero,” also intended to shoot any law enforcement officers who tried to take away their weapons at Lakeshore High, according to the Sheriff’s Office.

The three created detailed plans for obtaining firearms and taking them to school on Monday, Strain said, adding that he and his investigators felt the threat was real.

“We are convinced that these people fully intended to show up at their first day at school fully armed,” Strain said.

The youths “fully admitted not only the planning but the intended carrying out of the plan,” the sheriff said.

Asked if the three 15-year-olds had an escape plan, Strain replied they planned to end their own lives after the shootings.

All three were booked into the Florida Parishes Juvenile Detention Center on counts of conspiracy to commit terrorism. Their bond hearing is scheduled for Monday afternoon, according to the Sheriff’s Office.

If any of the three are released on bail Monday, the school system will take appropriate action, said Meredith Mendez, director of communications for the St. Tammany Parish School System.

The disciplinary actions the school system could take in this case include suspension, transfer or expulsion, she said.

Law enforcement authorities became involved in the case after a student informed school officials of concerns about the “Day Zero” group’s intentions, Strain said. School officials contacted the Sheriff’s Office, which in turn opened an investigation June 17 into the matter.

Search warrants resulted in collection of evidence and confiscation of the boys’ computers, the sheriff said.

“Intelligence that was gathered led me to believe, and our investigators to conclude, that this was a valid and serious situation,” he said.

“The planning, we believe, occurred throughout the summer,” Strain said. “We seized a good bit of their planning documents.”

Asked if the students had any maps or timetables, Strain replied, “there was a great deal of detail.”

The sheriff said he believes all those involved in the planning are in custody.

“We cannot say right now that there were absolutely no other members” in the group, but all evidence indicates that all of the students who intended to participate in the shootings have been arrested, Strain said.

The Sheriff’s Office will continue to obtain intelligence information and will beef up security at Lakeshore High and at other parish schools, Strain said.

Many of the steps taken won’t be the types of things students would see, he said.

That additional security and information-gathering won’t just be for one day, but will be ongoing, he said.

Asked for details about the students, he said one is from the Mandeville area and two are from the Lacombe area, but he is not aware of any of them “being on our criminal radar.”

Mendez said school officials are not allowed to say whether the students had histories of disciplinary problems.

Things worked as they should in the reporting and handling of the case, Strain said.

“Everything worked perfectly,” he said, adding that he thinks school and law enforcement authorities managed to prevent “an incredible and devastating attack on the school.”