AUGUSTA — A Waterville man will spend three and a half years in prison for accosting a clerk in an adult bookstore by wearing only his socks and sneakers.

The man, Scott L. Thompson, 41, left the video viewing area of the store and was masturbating at the time, according to an affidavit filed with the court.

Thompson was convicted in 1991 of murdering his mother and he has previously pleaded guilty to indecent conduct in connection with the Sept. 4, 2011, incident at the First Amendment Adult Store in Waterville.

This morning, Thompson was in Kennebec County Superior Court to be sentenced on that charge as well as charges he violated his probation by committing that crime.

Thompson was on probation for breaking into Treasure Chest II in Waterville and stealing novelty items in July 2009, and for groping a woman in a Waterville store later that year. He had been on probation for two months when he was arrested Sept. 4, 2011, at the First Amendment store.

He previously spent almost 20 years in prison for stabbing his mother to death in their East Winthrop home. The murder occurred in 1989 after Thompson, who was 17 at the time, was high on marijuana and had watched pornographic movies, according to published reports.

On Tuesday, Justice Michaela Murphy sentenced Thompson to six months behind bars on the indecent conduct charge — the maximum sentence for the misdemeanor — and ordered him to serve it after he is finished serving the three years that had been suspended on the burglary charge.

“We requested the maximum sentence that was available,” said Acting District Attorney Alan Kelley. “This was a situation where we view Mr. Thompson as being very dangerous. Certainly it would be beneficial to have him supervised when he’s released, but he’s demonstrated there’s no way of protecting the public when it comes to him.”

In a sentencing memo, Thompson’s attorney, Kevin Sullivan, told the judge Thompson was at the store “doing there was he was invited to do, but he overstepped his license by walking out to the front part of the store while having his genitals exposed, causing alarm to the store clerk.”

Sullivan asked that some probation remain so Thompson would be supervised when he is released. He said the more recent crimes occurred because Thompson was released without support or aid after serving a lenghty sentence for murder.