US grandmother discovers grandson's plan for school shooting

A US grandmother may have prevented another school massacre by reporting her grandson to authorities.

Following the shooting at Marjory Stoneman Douglas High School on Wednesday, police called for people to report any suspicious behaviour they see online or in their homes.

Joshua O'Connor seemed to be planning on shooting up a high school in Washington, but then his grandmother Cath O'Connor intervened.

She called police to her Washington home after she discovered disturbing notes in the 18-year-old's journal.

"I'm finding journal entries from my grandson. He's planning to have a mass shooting at one of the high schools," she said in a call to authorities.

An excerpt from the boy's journal reads:

"I'm preparing myself for a school shooting. I can't wait. My aim has gotten much more accurate."

O'Connor was in possession of a semi-automatic rifle which he had hidden in a guitar case, as well as bomb-making equipment and a detailed plan to shoot people at Aces High School where he was a student.

"I need to get the biggest fatality number I possibly can," he wrote in his journal.

Also revealed in his journal was that he had committed an armed robbery at a local store earlier in the week, using his semi-automatic rifle.

Authorities believe the robbery was to gain more funds to buy ammunition for the planned school shooting.