Olathe ? A suburban Kansas City social worker was almost decapitated in a chain saw attack during a home visit, prosecutors said Tuesday in opening arguments in the trial of the man accused of killing her.

Defense attorneys and prosecutors offered new details about the August 2004 death of Teri Lea Zenner, 26, a Kansas University graduate student who worked for the Johnson County Mental Health Center. She was killed when she went to Andrew Ellmaker’s Overland Park home after telling her husband she was making a short visit to make sure a client was taking his medication.

Ellmaker, 20, is charged with first-degree murder in Zenner’s death and with aggravated battery for injuries his mother, Sue Ellmaker, suffered while trying to protect Zenner.

During opening arguments Tuesday, prosecutor Sarah Geolas described the numerous wounds Zenner suffered, saying the social worker probably would have been decapitated if her hair had not become entangled in the chain saw’s blade.

Defense attorney Patrick Lewis did not dispute that Ellmaker killed Zenner. Instead, he argued that Ellmaker wasn’t planning on killing his social worker and that she was already dead from a stab wound to her throat when Ellmaker retrieved the chain saw from his bedroom. Jurors will be allowed to consider the lesser charge of second-degree murder, which doesn’t require that the prosecution proves premeditation.

“Andrew had absolutely no plan to kill Teri Zenner,” Lewis said.

“At the end of this case, we are going to ask you to find him guilty of what he did, but nothing else,” he told jurors.

During a pretrial hearing, the emergency room doctor testified that Ellmaker had been diagnosed with schizotypal, a personality disorder. But Ellmaker’s attorneys have decided not to use their client’s mental illness as a defense.

Geolas said Zenner’s visit started normally, with Ellmaker, who was on probation for marijuana possession, signing papers Zenner had brought. At some point, Geolas said, Ellmaker lured a reluctant Zenner to his bedroom. The social worker begged to be released, Geolas said, but Ellmaker had a weapon, which he later described to police as “the knifiest looking knife.”

For 30 minutes, Zenner was in Ellmaker’s bedroom, Geolas said. Then, his mother returned from the store, heard the social worker crying and threatened to call police if her son didn’t release Zenner by the count of three, Geolas said.

At the end of the count, Zenner rushed down the stairs with blood spurting from the neck wound and Ellmaker following behind her.

Sue Ellmaker threw herself between her son and Zenner, Geolas said, and was stabbed in the back multiple times. If the knife hadn’t bent in her back, giving her a chance to flee to a neighbor’s house and call 911, Sue Ellmaker might have died, the prosecutor said.

With his mother gone, Geolas said, Andrew Ellmaker went to his bedroom, turned on loud music, grabbed his chain saw and began cutting into Zenner.

Later, Geolas said, Ellmaker started a fire after pouring gasoline around the house and on Zenner’s car, then fled on foot.

When Ellmaker was arrested near his home, he had a pellet gun and another weapon, the prosecutor said. Without prompting, he told the police officer who arrested him: “I just killed my therapist with a chain saw,” Geolas said.

Lewis pointed out that the attack occurred after the second day of classes at Ellmaker’s high school, where the then-17-year-old had obtained only enough credits to be a sophomore. He had spent much of those first two days hiding in the bathroom, apparently unhappy about being mainstreamed with the general student body after spending the previous year in a special education classroom.

Lewis also said Ellmaker was suicidal and had taken about 60 pills, which he threw up inside and outside the patrol car after his arrest. He said the doctor who examined him noted his affect was “flat” and the teen initially was placed on suicide watch at the juvenile detention center.

In testimony Wednesday morning, neighbor Sarah Vogelsberg recalled Sue Ellmaker’s frantic knocks on the door and said the woman was covered with red specks.

In the 911 call from Vogelsberg’s home, which was played for jurors, Sue Ellmaker is heard begging for help.

“You’ve got to hurry,” she said. “She’s dying. I’m telling you she’s dying.”

The state’s case is expected to take two days to present.