COLVILLE, Wash. — An 11-year-old boy was convicted on Friday of conspiracy to commit first-degree murder in a fifth-grade plot against a female classmate.

Judge Allen Nielsen of Stevens County Superior Court said on Friday that “simple anger” had fueled the plot, which the boy devised this year with a 10-year-old classmate at Fort Colville Elementary School in northeast Washington.

Staff members seized a handgun and a knife that the boys took to school on Feb. 7.

The judge, who called the trial “the most serious of my career,” rejected defense efforts to portray the boy as unable to separate fact from fiction.

After the verdict, the 11-year-old defendant was led from the courtroom in tears. He is due back in court on Nov. 8 for a sentencing hearing.