TWO teenage boys have been charged after allegedly planning a massacre at Renmark High School this year, which police say could have led to multiple deaths.
One boy, 16, has been charged with solicit to murder, the other, 18, has been charged with four counts of aggravated threats.
Both are in custody and were students at the school.
South Australia Police allege the accused were planning an attack at the school involving explosives and knives.
Searches allegedly discovered knives, and homemade armour and bombs.
The matter first came to the attention of police on November 3, with both taken into custody soon after.
"Police will allege they intended to cause harm to a large number of people," Detective Superintendent Inspector Des Bray said.
"There is no doubt in my mind that we prevented a catastrophe."
He said police will allege the pair were involved in planning the attack between June 1 and November 7 this year.
Riverland police first received information on November 3 about a possible attack on a school, Det Supt Bray said.
"After investigations, on November 7, they conducted a search on the juvenile's home, and found him in possession of prohibited weapons, homemade body armour and materials that could be used to make bombs."
Det Supt Bray said the pair were not going to target specific individuals "but the faculty and students".
"They told some other people about what they were intended to do, and then they told other people.
"Soon it became known to quite a number of people.
"We believe most likely the attack would have occurred, if not stopped, before the end of the current school term."
"There is no evidence to show that any other person was linked with this matter, nor did the two individuals have possession of firearms."
Det Supt Bray said it would be "unbelievable to think that this sort of thing could happen in SA."
"I am not aware of us having an incident like this in recent times," he said.
He said police believed the duo was "actively planning, it was a real threat, and it was likely it would have been carried out if it hadn't been thwarted by the police and actions of the community".
Renmark High School principal Mat Evans wrote to students and parents about the matter on Wednesday afternoon.
"I wish to reinforce that the Renmark High School community is safe and remains safe," he said.
"Also can I state that all action was taken as soon as possible once concerns were raised.
"I thank each and every one of you for your ongoing support of our school community and the very positive way in which we have dealt with this difficult situation."
Det Supt Bray said police took the rumours seriously.
"When you hear these sort of things getting bandied about it, it warrants police taking it seriously, and even though it might be rumour."
With both teenagers detained, there is no risk to the community, he said, and no evidence to suggest anyone else was involved.
Police have worked closely with a number of other organisations during the investigation, particularly the Department of Education and the school authorities, he said.
A community forum was recently held in Renmark, 254km northeast of Adelaide, in relation to the incident.
"The safety of the community has been, and remains SAPOL's highest priority," Detective Superintendent Bray said.
"If there is any risk to the community, SA Police will take appropriate steps to advise the community in a timely manner."
The offence of solicit to murder carries a maximum penalty of life imprisonment.
Anyone with information that may be of use to police, and have not already been interviewed, is asked to call Crime Stoppers on 1800 333 000.