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HOUSTON (AP) — A Texas woman pregnant with her fourth child has been charged in the deaths of her two infant sons in 2011 and earlier this year, police said Tuesday.

Kristi-An Walker, 20, was indicted last week on a murder charge and two counts of injury to a child, said Angleton police Sgt. Kirk Coleman. Investigators say she told them she became frustrated with the children when they wouldn't stop crying.

Angleton officers looking into the June death of Walker's 6-month-old son learned details about how another one of her sons, a 4-month-old, had died in April 2011 in nearby Freeport. It's not clear whether a criminal investigation was done in that child's death, and Freeport police did not immediately return a phone call for comment Tuesday.

The murder charge is in connection with the death of the 6-month-old, and the charges of injury to a child are for both deaths. Coleman said Walker is accused of banging her 6-month-old son's head against a door jamb at her home in Angleton, about 44 miles south of Houston. He said the 4-month-old also suffered head trauma, which investigators learned was from Walker allegedly slamming the child hard against his baby seat and the metal part of his bassinet.

"She said she did it on both occasions because she was frustrated with the kids crying and she couldn't deal with the constant crying," Coleman said. Officials with Child Protective Services still were compiling details on the two infants' deaths Tuesday and said they would not have information until Wednesday at the earliest.

Walker remains jailed on a $1 million bond. Court records did not list an attorney for her. She also has a 3-year-old son who has been placed in foster care by CPS, agency spokeswoman Gwen Carter said.

Coleman said Walker banged the 6-month-old's head on June 20 but didn't call for medical help. He said someone called 911 early the next day, and an ambulance was sent to Walker's Angleton home. The baby died at a hospital and police began their investigation.

Coleman would not say who made the 911 call. He said investigators learned the child had suffered head trauma after talking to Walker. "She confessed she started the abuse when he was 4 months old," the sergeant said.

Angleton police arrested Walker on a charge of injury to a child on July 26. She has been jailed since then. Walker is now at least 10 weeks pregnant. Coleman said jail officials are monitoring her closely.

He said Walker showed "no real emotions" when telling investigators about her sons' deaths.