By BOBBY ROSS JR. Associated Press Writer

PLANO, Texas (AP) - A 33-year-old mother was charged Friday in the slaying of her two young daughters, whom a neighbor recalled as "happy, playful" girls who liked chasing a pet dog around the back yard.

Lisa Diaz was arrested Thursday night after her husband returned from work and found 6-year-old Briana and 3-year-old Kameron dead in a bedroom, police said.

The cause of death remained undetermined Friday afternoon, the medical examiner's office said.

Police Capt. Harry Manning, commander of the criminal investigations division, said Diaz had not given police a written statement.

"We do not have a confession from her at this time," Manning said Friday. "I cannot get into the specifics of it, but there were statements made at the scene that indicated enough probable cause for us to go ahead and arrest her."

Diaz met her husband in the garage about 6:20 p.m. Thursday and told him the girls were injured, police spokesman Xavier Badillo said. The father discovered his daughters lying on a bed and called 911.

Plano police late Friday released the 911 tape, in which Angel Diaz is heard frantically screaming "please help me" and "something bad happened to them."

Diaz told the operator his daughters weren't breathing. "I don't know what ... my wife did," he said.

On Friday, 42-year-old Angel Diaz spent the day mourning with his family.

"I'm just grieving here with my family. We really don't know what else to do," he told The Dallas Morning News for its Friday online editions.

The mother, bleeding from an undisclosed injury, was taken by ambulance to a hospital, where she was treated and released, Badillo said. She was in the Collin County Jail in McKinney on Friday "under close observation," sheriff's Lt. John Norton said.

Police said medical privacy laws prevented them from discussing the nature of the mother's injury. Asked if it appeared self-inflicted, Manning said, "The initial observations at the scene would indicate that is a possibility."

The family had lived in the one-story, yellow-brick house in a working-class neighborhood since 1999, and the only call to authorities involved a medical situation in May 2001, police records show.

"From what we know, there is no history with Child Protective Services," Badillo said.

Investigators declined to discuss whether they found evidence of a struggle. Asked if the husband had given any indication of a history of mental illness or other problems involving the wife, Manning did not respond directly.

He said police had talked briefly with the husband and Lisa Diaz's 16-year-old daughter, who lived at the house and arrived about 10 minutes after officers got to the scene.

"They were interviewed to some degree ... but again, remember that they did suffer quite an emotional event," Manning said. "So we will continue with that as we're able."

Michelle Marroquin is the niece of David Sanchez, the 16-year-old's father. Marroquin recalled a family gathering in August.

"I spoke to Lisa for a few minutes," Marroquin of Flower Mound told The Morning News. "She seemed happy. She was laughing and talking to everyone in the room. She told me she wanted to have more kids."

Badillo said there was blood in the room where the children were found and that the mother was bleeding.

"I just know that there was blood and the children succumbed to injuries," he said. "The father appeared, to me, pretty upset."

The homicides were the first this year in Plano, a suburb north of Dallas.

"My heart just sank," said Saif Quershi, 14, the neighbor who recalled seeing the girls play with the dog. "I was just shocked. How could this happen, especially in this neighborhood?"

Neighbor Anthony Smith said he often saw the mother pushing Kameron in a stroller or picking up Briana from nearby Mendenhall Elementary School.

"She just seems like a regular mom. She seems to be a normal person," Smith told The Dallas Morning News. "She's very quiet, though."

School was closed Friday for parent-teacher conferences.

District spokeswoman Nancy Hall said the district would send home letters Monday to parents and make counselors available to students on an individual basis. No public mention of the death will be made, given the children's ages, she said.

Superintendent Doug Otto issued a statement saying the district's staff and students were "saddened by the tragic death."

"We join in sending our sympathy to the family and will work together to support the staff members at her school and the family," Otto said.