The boys, ages 10 and 11, were expected to be formally charged in juvenile court with conspiracy to commit first-degree murder, possession of a firearm and witness tampering for allegedly bringing a stolen gun and a knife to school and threatening to kill a number of classmates, The (Spokane) Spokesman-Review reported.
The boys at Fort Colville Elementary School were taken into custody Thursday after a search revealed one boy had a knife and a handgun in his backpack.
During interviews, police learned the boys intended to use the weapons Thursday, Stevens County Prosecutor Tim Rasmussen said.
"These young men conspired to kill," Rasmussen said. "It was interrupted by the bravery of a fourth grader who saw something and said something ... and interrupted a murder."
Colville School District Superintendent Michael Cashion said the plot was discovered before school started Thursday, The Spokesman-Review reported Friday.
"We've been told that the boys had a plan to kill an ex-girlfriend ... and harm other students," Cashion said. "There was no list, but names were given to the police."
"I can't get my mind around it," he said.
During last week's hearing, Deputy Stevens County Prosecutor Lech Radzimski asked the judge to order the boys not to have contact with eight children identified by their initials.
Fort Colville Elementary Principal Clayton Allen said all of the alleged victims were in the same 5th grade class.
The judge set a Feb. 20 hearing to discover whether the boys had the capacity to form criminal intent, Radzimski said.