PORTLAND — Prosecutors rested their case today at the murder trial of Chad Gurney.

Their final witness was 20-year-old Amber Wallace, one of the defendant’s former girlfriends. She testified that Gurney invited her over to his Cumberland Avenue apartment on May 25, 2009, the same day he killed Zoe Sarnacki.

Wallace said her relationship with Gurney had never been exclusive because he wanted to see more than one woman, and preferred to be with women several years younger than him.

Wallace was aware that Gurney, who was 27 in the spring of 2009, was spending time with the 18-year-old Sarnacki. But Wallace did not know Sarnacki was at his apartment on May 25. Wallace said Gurney sent her several text messages inviting her to come to the apartment after she got out of work that night.

When Wallace arrived at Gurney’s apartment after 9 p.m., there were several police officers and firefighters around the building. Police took Wallace to police headquarters to question her about Gurney, who was missing at the time.

When he turned himself in to police the next morning, Gurney admitted to strangling Sarnacki, mutilating her body and setting her on fire inside his apartment.

Wallace said Gurney could be mean and withdrawn, but he had never been violent with her.

“The person I knew, I didn’t think that he would do that,” Wallace said.

She also testified that Gurney had never talked to her about hearing voices, and there were no indications that he was ever delusional or suffering from a serious mental illness.

Wallace visited Gurney four or five times in jail after his arrest. She testified that she asked him what would have happened if she, and not Sarnacki, had been in the apartment on the afternoon of May 25. She said Gurney told her “that if I was the one who was there, nothing would have happened to me. He made reference to me being bigger than Zoe.”

After Wallace’s testimony, the defense team began calling witnesses, starting with two people who knew Gurney through the tattoo scene in Portland’s Old Port.

Gurney has pleaded not criminally responsible by reason of mental disease or defect.

Next week, doctors from the State Forensic Service are expected to testify that Gurney, though troubled, was legally sane when he killed Sarnacki.

An independent doctor hired by the defense is expected to testify that Gurney was delusional at the time and met the criteria for being legally insane.

11:06 a.m.

PORTLAND — A former girlfriend of Chad Gurney was on the stand this morning, on the fifth day of his murder trial at Cumberland County Superior Court.

After her testimony, the prosecution was expected to rest its case against Gurney, 29, who is on trial for the May 25, 2009, killing of 18-year-old Zoe Sarnacki.

Then it will be the defense team’s turn to call witnesses and begin to build its case that Gurney was legally insane when he killed Sarnacki.

The state’s position is that Gurney was sane at the time.

Most of the evidence presented by the prosecution during the first week of the trial, which opened Monday in Cumberland County Superior Court, was undisputed.

Gurney’s lawyers concede that the 29-year-old defendant strangled Sarnacki, mutilated her and set fire to her body in his apartment on Cumberland Avenue in Portland on May 25, 2009.

Most of the witnesses who have testified have confirmed those facts and developed a timeline of events.

That means the heart of the case — the differing opinions of doctors who evaluated Gurney in the weeks and months after the killing — has yet to be heard by Justice Roland Cole.

Because Gurney waived his right to a jury, Cole alone is responsible for hearing the evidence and returning a verdict.