BROCKTON - A Plymouth Superior Court judge wasted no time rendering a decision yesterday morning against Joseph Nee, a former Marshfield High student accused of plotting a Columbine-style ambush on the school.

The 21-year-old sat silently about 11:30 a.m. as Judge Charles M. Grabau found him guilty of conspiracy to commit murder and acquitted him of two other charges.

It was the second time that morning the judge proclaimed Nee's guilt.

Much to the surprise of Nee's lawyers, Grabau started to issue his ruling close to 10 a.m., before closing arguments had been made. "The Commonwealth has proved this case beyond a reasonable doubt . . ., " he began.

Several people gasped. Some of Nee's family and friends began sobbing. Nee's father, Thomas, stormed out of the crowded courtroom, red-faced, muttering "This is due process?"

But when Nee's lawyer, Thomas Drechsler, stood up and pointed out that he had not made his closing argument yet, the judge stopped short of announcing his decision and directed the lawyer to begin his closing argument.

Almost two hours later, following Drechsler's impassioned argument on behalf of his client, the judge issued his ruling, convicting Nee on one of three charges. Nee is scheduled to be sentenced Tuesday.

Inside the packed courtroom yesterday, Nee embraced his sobbing mother, telling her: "It's all right. It's all right."

It was an emotional and dramatic conclusion to a four-day bench trial that included testimony from a dozen witnesses. It also marked the end of a disturbing case that sent shockwaves through the town of Marshfield in the fall of 2004, when Nee and his friend, 16-year-old Tobin Kerns, were accused of conspiring to kill students and faculty at Marshfield High.

Authorities learned about the plan in September of that year, when Nee went to police with two classmates and told officers that Kerns was planning a massacre at the school. Nee told police the plan involved taking ammunition and explosive devices into the school, securing the school's exit doors with bicycle locks, and shooting students and staff.

Police arrested Kerns the following day.

Police didn't arrest Nee until a month later, after friends of Kerns implicated Nee as the mastermind of the plot. The two youths were once close friends; Nee even lived at the Kerns's home for a month during the spring of 2004.

Kerns's father, Ben, said that the boys had a falling out and that he believed Nee was trying to frame his son.

A grand jury returned indictments against Nee and Kerns in October 2004, charging both with conspiracy to commit murder, promotion of anarchy, and threatened use of deadly weapons at a school. Kerns and Nee pleaded not guilty to the charges.

Kerns was tried and found guilty of threatening to use deadly weapons and conspiracy to commit murder. In November, he was sentenced to 10 months in jail. He is being held at the Plymouth House of Correction.

In his closing argument yesterday, Drechsler cited a lack of physical evidence in Nee's case and inconsistencies in the prosecution witnesses' testimony.

Nee never planned to take part in the alleged plot, Drechsler said.

"My client was the first one who went to the police," he said. "Do the actions of Joe Nee bespeak of someone who intended to attack the school and kill people?"

Assistant District Attorney Karen O'Sullivan gave the judge the prosecution's point of view.

"The defendant's motive for going to police was to save himself," she said. "The defendant did not have intimate knowledge [of the plan] because he had overheard Mr. Kerns; the plan was as much his as it was Kerns's.

"This was more than a couple of teenagers shooting their mouths off," she said.

Grabau could sentence Nee to as much as 20 years in prison or let him go. Since January 2005, Nee has been free on $20,000 cash bail.

He is currently a student at Bunker Hill Community College.

Lawyers on both sides of the case said they were satisfied with the judge's ruling.

Plymouth District Attorney Timothy J. Cruz said he was pleased Nee was convicted on what he believed to be "the most serious" of the three charges.

"I'm hopeful the town of Marshfield will be able to heal from this," he said. "I'm hopeful that the two young men charged with these crimes will get their acts together and try to be good citizens."

Emily Sweeney can be reached at esweeney@globe.com.

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