Houston – A Houston mother who calmly told an acquaintance that she had killed her two children and hid their bodies underneath a neighbour's home, drowned them in a bathtub by forcibly holding their heads underwater, authorities said on Monday.Sheborah Thomas, 30, was charged on Sunday with capital murder of a person under age 6, according to court records. She remained jailed without bond.According to a probable cause affidavit read during a court hearing before a magistrate judge on Monday, Thomas told police that after she picked up her 7-year-old son and 5-year-old daughter from daycare on Friday, she fed them and then prepared a bath for them. Thomas first called her daughter into the bathroom."She grabbed (the daughter) by the head and forced her underneath the water until she stopped moving," according to the affidavit. "After she wasn't moving and she wasn't breathing, she took the body out of the water and placed her in the bedroom."Thomas then called her son into the bathroom and she placed him in the water, authorities said. The boy struggled against his mother, grabbing her hand, but Thomas "continued to force his head underneath the water until he stopped struggling, he stopped moving," according to the affidavit.The mother placed his body next to his sister's in the bedroom and then later on Friday put their bodies in a trash can behind her home, the affidavit said.'So matter-of-fact'Thomas was not in court for the reading of the affidavit. Online criminal court records do not list an attorney to comment on behalf of Thomas, whose next court appearance is set for Wednesday.Police say investigators are still trying to determine a motive, but it is believed that Thomas acted alone. Autopsies have been ordered.On Sunday, Thomas tried to first bury the bodies next to her house but was not able to dig a deep enough hole, authorities said. She then rolled the bodies underneath a neighbour's house, according to the affidavit.Thomas then began packing up her belongings and enlisted an acquaintance who saw her dumping things in a field to help her move. When the man asked about her kids, she calmly said she had killed them, said Houston police spokesperson Kese Smith."She was so matter-of-fact about it, he didn't think she was serious. He thought she was joking," Smith said. "He continued to help her pack."Smith said the man eventually realised something was wrong when he asked again and got the same answer, so he drove the woman to a police precinct. Investigators don't yet know whether the mother has a history of mental problems, Smith said. He said police had been to the home before but for "nothing major".In 2012, Child Protective Services (CPS) had placed the two children with their maternal grandmother after the girl, then two years old, was found wandering the streets with an intoxicated homeless man.'Seemed happy'In court filings, CPS officials said Thomas, who had a history of drug use, and her children had been homeless at the time and were staying at a shelter.The children were returned to their mother's custody the following year, and in 2014, after the children had been back with their mother for 10 months, CPS officials said the kids "have been doing fine".Texas Department of Family and Protective Services spokesperson Tejal Patel said a "top to bottom review" of the family will be conducted.Shirley Baines, 66, who lives a few houses down from where Thomas and her children lived, remembered giving the children popsicles and thinking that Thomas "seemed like a happy person". Baines said she and other neighbours were shocked by the deaths."That just don't make sense. That's just horrible," she said.Houston police say Thomas also has a 12-year-old son, but he was with his father.According to court records, Thomas has several misdemeanour convictions, including for theft and failure to identify oneself to an officer.