He was supposed to be watching his girlfriend's two children Friday night, but now he's accused of abusing them and killing the 3-year-old girl.
It appears he may have called police not realizing the child was dead.
The 18-year-old suspect , Martwane Mackey, called police from a pay phone at 2:00 a.m., telling them 3-year-old Tyshay Austin wasn't breathing.
When police arrived at the scene near 15th Avenue and Missouri, the child was in his arms and it was too late. The child's injuries were too severe.
"This is trauma that is clearly child abuse," said Phoenix Police Sgt. Steve Martos.
Officers then went to the apartment just across the street from the pay phone and found another child - a 1-year-old girl - also showing signs of abuse.
"He had admitted involvement in this case. Admitted to abusing both of these children," said Martos.
Because it happened so early in the morning, most of the neighbors woke up and saw all of this.
"I mean it is a child..she hasn't even had a chance to live," said an area resident.
The suspect had been dating the 18-year-old mother of the two children for about six months. Police say she had to work late and asked him to babysit.
"That really touches you..because now you are grown and that little innocent child can't do nothing," said Ray Cleveland, a neighbor.
The mother was asleep when the boyfriend called police. She told officers she had no idea what happened, but they'll have some questions for her too.
"If there was some involvement that she had in abusing either one of these children. Two, if she did have any knowledge. Third, if she did have any knowledge of any ongoing abuse, why didn't she report any of the abuse to authorities?" said Martos.
The 1-year-old child, Zariah, remains in a hospital. Police say she is going to be okay.
Mackey faces child abuse and murder charges.
The children's grandmother, Rhonda Brown, says her daughter would not have anything to do with any abuse of her daughters and had no idea that Mackey would hurt them either.
"What can a 3-year-old do that you would have to beat them to death?" said Brown. "What would a 1-year-old do that you would have to hit them in the face to where their eyes are swollen and puffed up out of their face..what did they do?"
Brown doesn't know Mackey, but she does know her daughter. She told FOX 10 that there is no way that she would've allowed her children to be hurt.
"I know my daughter would not have just left these kids with anyone..knowing that they would be hurt," said Brown. "Death would not be good enough for him, sorry, it's just not. This was a 3-year-old child."
Brown says the last time she saw her two little granddaughters was several months ago. She is trying to repair her relationship with her daughter.
To donate to the family: Ty'Shea and Zariah Memorial Fund, Wells Fargo Acct # 9373518860