When Roy Kipp suspected his wife of infidelity, his thoughts turned to murder, witnesses in the retired officer's double-murder trial testified Tuesday.

Less than a week before he gunned down his wife and her lover, "he told me that if he ever caught her with anybody, he'd kill her," Sandy Kipp's sister, Wendy Postan, told jurors in a courtroom in Punta Gorda, Fla.

Sandy Kipp's sister, Wendy Postan, leaves court in tears

Prosecutors contend Roy Kipp, a former sheriff's lieutenant, shot his wife and her lover, Collier County sheriff's deputy Jeff Klein, because she had left him.

Lawyers for Kipp say he acted both in the heat of passion after discovering the pair in a compromising position and in self-defense after his former friend Klein wielded a gun. The 44-year-old faces the death penalty if convicted of first-degree murder.

After emotional testimony from Postan and Sandy Kipp's cousin and best friend, Kathy Miles, the prosecution rested Tuesday. Kipp's defense will begin Wednesday morning and the jury could begin weighing evidence in the case Friday.

Postan testified that a week before the killings she called her sister to wish her a happy Mother's Day. Roy Kipp answered the phone and quizzed her about whether Sandy was having an affair.

"He asked me if I knew of any boyfriends," Postan recalled. When she said no, he threatened to kill Sandra if he discovered otherwise.

Kathy Miles, Sandy Kipp's cousin and best friend

Miles followed Postan to the stand and said the Kipp's had marital problems extending back five years. The week of the killings, Miles said, she helped Sandy Kipp move into a new apartment about a five minute drive from the family home she shared with Roy and her daughter, Danielle.

Miles said Roy Kipp peppered her with questions suggesting his wife was cheating.

"He said, 'Well, if I find out she's doing this, I'm going to kill somebody,'" Miles said.

But, she added, "he said he would never do anything to hurt [Sandy] or Danielle."

On cross-examination, Miles acknowledged that Klein and Kipp were having an affair and that as far as she knew, Roy Kipp never knew about it until the night of the shooting. The defense has said Kipp was so shocked to discover his wife and friend having an affair, he could not control his actions.

Defense lawyer Michael Orlando also pressed Postan about the Mother's Day conversation. If the exchange was so disturbing, the defense lawyer asked, why hadn't she warned her sister?

"I didn't know what to do," Postan replied through tears.

Also Tuesday, prosecutors recalled medical examiner Manfred Borges to the stand. On Monday, Borges grudgingly admitted that a ridged mark on Klein's hand might have been made by a gun which was later removed from the crime scene.

But Borges said that after reviewing photos overnight, he had determined that the waistband of Klein's shorts, not a gun, made the mark.

Because of extensive media coverage in Collier County, the case is being tried in Charlotte County, 50 miles to the north. The trial is being covered live on Court TV.