DOVER, N.H. – A 20-year-old Somersworth man was convicted Thursday of first-degree murder for stabbing his pregnant ex-girlfriend to death a year ago.

Anthony O’Leary’s lawyers contended the killing was a crime of passion that constituted manslaughter, not premeditated murder, which carries a mandatory sentence of life in prison without parole. But a Strafford County Superior Court jury disagreed.

Treasure Genaw, 17, of Berwick, Maine, was three months pregnant with O’Leary’s child when O’Leary stabbed her repeatedly last June 7. He had asked her out for ice cream and a chance to talk.

Genaw’s family and friends had waited anxiously outside the courtroom during about 101/2 hours of deliberations over three days. Most wore butterfly pins, pendants or necklaces in honor of Genaw, whom they say loved butterflies.

O’Leary confessed to stabbing Genaw repeatedly and leaving her body off Route 236 in South Berwick, Maine. He was caught that night by Massachusetts State Police after a high-speed chase.

Prosecutors said O’Leary killed Genaw because he could not accept her ending their relationship. O’Leary’s adoptive mother, Amy O’Leary, testified that he called frequently to talk after Genaw broke up with him in May 2004.

“He told me he didn’t understand why she didn’t want anything to do with him anymore,” she said.

She said her son called her the day of the killing and said Genaw had stolen $500 from his apartment and he had no money to pay rent or buy food. He also was concerned that he would not have a role in his baby’s life, she said.

He called back that night, she testified, and said he had “just confessed to murder … that he stabbed Treasure in a jealous rage.”

In police interviews, O’Leary said the stabbing grew out of an argument.

“We were arguing and fighting about stupid stuff,” he said. “I don’t know what I’ve done, it happened so fast.”

“It wasn’t supposed to go that far, I usually just walk away,” he added. “Something in me snapped.”

But Assistant Attorney General Charles Keefe told the jury the killing was deliberate.

“She tried to escape, but the defendant chose to hold on to her,” he said in closing arguments. “She tried to fight back, but the defendant’s strength was overwhelming. Finally, with her life bleeding out of her, she begged for her life.

“Anthony O’Leary’s response was, ‘It’s too late.’ It was deliberate, without mercy.”

Genaw was stabbed nine times – in the eye, throat, abdomen and chest.

O’Leary claimed Genaw tried to burn him on the arm with a cigarette and that she picked up a utility knife and slashed at his fingers. He said he grabbed the knife and began stabbing her.

Keefe rejected the claim and police testified they saw no burn mark on O’Leary’s arm.