AUGUSTA — A convicted killer committed to the Riverview Psychiatric Center pleaded guilty Friday to escaping from the custody of the facility by not returning while he was out on a two-hour pass in 2018.

Mark Gessner

Instead of returning from his authorized, unsupervised time off from the Riverview campus within that two-hour timeframe, Mark Ian Gessner was accused by police of going to the Bath home of his father and threatening him with a knife, resulting in him being charged with two counts of criminal threatening. However, one of those charges was dismissed and he was acquitted, at trial in Sagadahoc County, of the other charge.

State officials later filed an escape charge against him, for which he was indicted in March, for not returning to the Riverview campus in a timely manner while on his two-hour pass in the 2018 incident.

He pleaded guilty to that charge Friday and was sentenced to 209 days imprisonment, which counts as time he already served. But it was a conditional guilty plea that Justice Michaela Murphy noted preserves Gessner’s right to appeal the case to the Maine Supreme Judicial Court.

“You’ve reserved your right to challenge, whether or not this charge of escape subjects you to double jeopardy,” Murphy told Gessner, who joined the court by video feed from Riverview on Friday.

The U.S. Constitution’s double jeopardy clause prohibits people from being prosecuted or punished twice for the same crime.

Murphy said she ruled against Gessner’s motion to dismiss the escape charge, but his plea reserves his right to appeal that decision. She said if Gessner prevails with his appeal, the conviction could be withdrawn.

Gessner, 56, was convicted of the 1994 murder of Melvin Henderson, a florist in Bath, for which he was sentenced to 30 years in prison. While serving his sentence for that crime in prison, Gessner was accused of aggravated assault and trafficking in prison contraband. He was found not criminally responsible for those charges and committed to the custody of the Commissioner of Health and Human Services in 2011.

He completed his murder sentence in 2016 after getting credit for “good time,” allowing him to be released after serving some 22 years. However, because he had been committed to state custody for the charges made against him in prison, rather than being released as a free man in 2016 when he left prison, he instead was committed as a forensic patient to Riverview Psychiatric Center. A forensic patient is someone who’s been involved with the criminal justice system or committed a crime but found not responsible for their actions because of insanity.

The Maine Supreme Court in 2017 upheld a lower court’s ruling that Gessner should not be released into the community.

At Riverview, Gessner twice petitioned the courts to be allowed to leave the state mental health facility with patient privileges in order to take part in community activities. A 2016 request was denied, but in 2018 he was granted up to two hours a day of unsupervised time as long as it was within 10 miles of Augusta.

In January he petitioned the courts to release him from state custody and be allowed to leave Riverview, but that request was denied. Court records indicate he has filed another such request, which is scheduled for a hearing in November.

Murphy said she found Gessner — whom she questioned about whether he understood the court process and his plea — to be of sound mind and body to make the plea.

His attorney, Clifford Strike, said Gessner understood what was going on and added that Gessner helped guide his defense and testified on his behalf in the Sagadahoc County trial, which found him not guilty of the charge he threatened his father.

Gessner has said he’s not mentally ill, delusional or psychotic. He further claims that state officials who evaluated him were lying in finding him not criminally responsible by reason of insanity following the 2011 charges in prison.