DALLAS (AP) - The bodies of two young sisters found dead last week in their Plano home showed no obvious signs of trauma, the Collin County medical examiner determined Monday.

The news came as county officials released a search warrant affidavit that revealed new details about the case against the girls' mother but provided little insight into how the girls were killed.

The deaths of 6-year-old Briana Diaz and 3-year-old Kameron Diaz have been ruled homicides but the medical examiner has not ruled on a cause of their deaths. That decision is expected to come later this week, said Plano police spokesman Xavier Badillo.

The girls' mother, Lisa Ann Diaz, 33, remained in the Collin County Jail on Monday, charged with capital murder in her daughters' deaths.

The affidavit by a Plano police officer says the girls were found nude under a sheet on a bed saturated with blood and other bodily fluids. Police found what appeared to be the handle of a knife and bloody clothing in the bathtub.

The affidavit says that when the girls' father, Angel Diaz, returned home Thursday his wife told him "something bad happened to the kids" and that she "didn't want them to suffer."

Later, Plano police Sgt. Randy Andrews found Lisa Diaz sitting in the passenger side of a vehicle in the garage. When he opened the door, the affidavit says, Lisa Diaz told him, "I don't know what I've done."

The affidavit suggests the youngest girl may have been a victim of sexual abuse and Capt. Harry Manning, commander of the criminal investigations division, said investigators were considering that possibility.

"We're not ruling out anything until we hear from the ME's office," Badillo said.

Meanwhile, Badillo said some officers in the Plano Police Department have sought counseling from chaplains as they struggle with the horror of the case.

"It's just not your typical type of homicide. When it comes to children, most of us have children ourselves. It's just very difficult on the officers," Badillo said. "We do have chaplains who are on call and we need to make sure that the officers are dealing with it emotionally and not repressing the emotions."