DALLAS -- A father who was under investigation by Child Protective Services for family violence has now been charged with killing his common-law wife's 4-year-old son.

Brandon Gordy, 27, is charged with capital murder in the July 14 death of Darwin Delgado.

According to his arrest warrant, child abuse unit detectives were called to Medical City Dallas Hospital after the child was found unresponsive and brought into the hospital. He died about an hour after he arrived.

Detectives at the hospital interviewed the boy's mother, Rosa Angon-Diaz, and Gordy, who was the only adult at home watching Darwin and his siblings when the incident occurred.

While speaking with detectives, Angon-Diaz told them that Gordy was watching the kids in their apartment at 8908 Park Lane while she was at work.

She also revealed that Gordy had been abusive with her the day before, grabbing her by the neck and throwing her up against a wall. She had visible bruises, the warrant states.

Angon-Diaz also told police that Gordy "has a quick temper" and that her son Darwin and his brother were afraid of him. Child Protective Services was already investigating Gordy for physical abuse against Angon-Diaz's other young son, the warrant says.

While being interviewed, Gordy told officers that he put Darwin in the bathtub with soap and left him for a couple of minutes to check on an infant in another room. He says he heard a loud noise in the bathroom and came in to find Darwin slumped over in the tub, partially face down in the water, and saw a bruise on the bridge of his noise and his left shoulder, the warrant states.

Darwin was unresponsive, Gordy said, so he performed CPR, called 911, and went to a neighbor's house for help. He also said he placed underwear on the child in case he went to the bathroom.

During an autopsy, it was discovered that Darwin's left clavicle was fractured and his rectum "suffered some type of trauma." Furthermore, Darwin had scalp contusions, brain swelling, and an injury to his lower lip that seemed to be in the "healing stages," the warrant states.

Two-thirds of his liver was lacerated, which caused his death. The report stated that injuries like those "could only occur from inflicted blunt force trauma," the warrant states.

Darwin's death was ruled a homicide.

Meanwhile detectives also interviewed two children, one being Gordy's biological son, who were home at the time of the incident. One said he never took a bath that day, like Gordy said, and overheard the suspect tell Darwin "I'm whooping your a**!"

The other boy, Darwin's brother, said he saw the suspect slap the victim in his mouth at one point.

When told about the boys' comments, detectives say Gordy denied any wrongdoing and "became belligerent."

The warrant states that it's the detectives' "professional opinion and belief that the suspect caused blunt force trauma to the complainant which resulted in his death."

Gordy is currently in police custody and charged with assault family violence - strangulation for abusing Angon-Diaz the day before. The murder charge bumps his bond amount up to $1 million.

CPS says it is working with law enforcement, and Darwin's siblings have been removed from the home and placed in foster care for the time being.