COLVILLE, Wash. -

Stevens County prosecutors were surprised Monday morning when a pre-trial hearing for one of two boys charged with conspiracy to commit murder turned into a plea hearing.

The 10-year-old boy, on advice of defense attorney Helen Hokom, pleaded guilty to a trio of charges before Judge Allen Nielson Monday morning, Stevens County Deputy Prosecutor Lech Radzimski confirmed.

The boy pleaded guilty to conspiracy to commit murder, juvenile in possession of a firearm and witness tampering charges. Judge Nielson asked the boy if he understood the charges he was pleading guilty to as well as the possible sentence he might face for his guilty plea.

The boy told Nielson about his plea change to guilty that, "I think it's best for my future. I know what I did was wrong."

Originally, the Monday morning hearing before Nielson was to be a hearing where the defense planned to argue for suppressing evidence in the case, specifically the gun found in the boy's backpack, because it was seized without a search warrant.

Instead, according to Radzimski, the defense brought a plea agreement to the hearing.

The standard sentencing in a case such as this is up to 120 weeks in juvenile detention, however there's a chance that Stevens County Prosecutor Tim Rasmussen  may seek an exceptional sentence.

Prosecutors now have 14 days to come up with a sentencing recommendation to present to the court.

The 10-year-old is currently in custody at the Martin Hall juvenile detention facility.

The boys were taken into custody by school administrators after another boy, Justin Sanders, noticed one of them playing with something in his backpack while getting off the school bus at Fort Colville Elementary School. Sanders saw a knife in the backpack and alerted a school official.

"I was watching him playing with his backpack. He got off, walking and slipped. Still playing with his backpack, zipper open, tripped and there was a bunch of papers and also a knife," Sanders said at a Colville school board meeting, where he was honored for helping thwart his fellow classmates' plot to kill another classmate.

"For one I'm proud, whole town is proud of this boy," Fort Colville Principal Clayton Allen said. "He made the right decision. He could've choose to walk away, easily ignore that. But, he knew that was wrong and had to tell somebody."

In fact, the day that Sanders spotted the knife in the backpack and alerted school officials was the day the two had been planning on making the attack on their classmate, according to Colville police Officer Scott Arms.

"Both explained to me that they had been planning this event about two weeks before that  date and on the date we made contact with them and interviewed these students, it was going to happen that day," Arms said.

The two boys were promptly expelled from school and taken into custody.

While the 10-year-old pleaded guilty Monday, the 11-year-old defendant was not party to the plea agreement. He also remains in custody at Martin Hall.