Henry Scholten, 29, pleaded no contest to the arson charges Wednesday in Sonoma County Superior Court and will be sentenced June 24.
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Each count carries a term of two, four or six years but Scholten has considerable credit for time already served.
Scholten's mental competence and his ability to participate in his defense have been at issue since his arrest on Feb. 22, 2006.
He was held to answer to the arson charges after a preliminary hearing but the case never went to trial.
Psychologists who examined him over the years gave conflicting opinions on Scholten's ability to understand the legal proceedings. He spent some of his time in custody at Atascadero State Hospital.
At one point, Scholten said he believed his deputy public defender had magical powers that would enable him to be freed and avoid a trial. On more than one occasion, he told court employees he just wanted to go home.
Scholten was charged with setting four fires at three unoccupied Healdsburg businesses within five blocks of each other between Feb. 15 and 19, 2006. The fires caused an estimated $500,000 in damage.
He was arrested after a woman told police she recorded a conversation with Scholten during which he admitted to setting the fires.