A former student convicted of plotting with another student to carry out a Columbine-style attack at Marshfield High School was sentenced Tuesday to nine months in prison.

Joseph Nee, the 21-year-old son of Boston Police Patrolmen's Association President Thomas Nee, was convicted last week of conspiracy to commit murder, but acquitted of two other charges. Because he was jailed for three months after his arrest, he will have to serve another six months, followed by two years of probation.

Nee faced up to 20 years in prison.

Nee was one of three Marshfield High students who went to police in September 2004 and reported that another student, Tobin Kerns, was planning to kill students and teachers in an attack similar to the 1999 rampage at Columbine High School in Colorado that left 15 people dead.

Kerns was arrested immediately, and Nee was charged a month later as a co-conspirator.

Kerns was convicted in 2006 of conspiracy to commit murder and threatening to use deadly weapons. He is serving a 10-month sentence.

Nee's attorney said the plan by four students who called themselves NBK -- "Natural Born Killers" -- was never a real threat, but prosecutors rejected that argument.

Authorities uncovered a hit list of students, school officials and police officers. They also found hand-drawn maps of Marshfield High School, computer files on bomb-building and a shopping list of weapons and ammunition.

Nee was 18 when he, Daniel Farley and Joseph Sullivan reported that Kerns was planning to kill students and teachers at the high school.

Prosecutors said Kerns and Nee were equal partners in planning the attack, which they said was meant to get back at people who had picked on them.

Nee's attorney, Thomas Drechsler, asked Judge Charles Grabau to spare Nee any additional jail time and sentence him to probation.

"My client did go to police, and Kerns didn't. I felt that he should have gotten a more lenient sentence," Drechsler said.

Drechsler said he plans to appeal the conviction based on Nee's renunciation of the plan before it was carried out.

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