Logan Lively was rushed to the hospital with severe head trauma last Tuesday.
She later died from the injuries.
Bonham Police Chief Mike Bankston said they determined the death was a homicide and arrested Michael-Chaz Allen Herman.
Herman is the boyfriend of Logan's mother.
Jake Lively, Logan's father, said he's certain Herman is guilty.
"I know he did it," Lively said. "He had to have done it."
Lively said he never met Herman. But he didn't trust him.
"Every time I'd ask if he was good with the kids or something like that, she'd tell me yeah, he was great with the kids, they love him. And that was what I had to go by," he said.
Bankston said Herman is charged with capital murder. He's also charged with injury to a child for allegedly abusing Logan's one-year-old brother, Colton, who is now recovering.
Herman's defense attorney said Herman maintains his innocence.
"We believe the charges against Chaz are premature and un-warrented. The investigation has just gotten underway and the autopsy hasn't even been completed yet," he said. "We won't know that for a few weeks."
Logan's father - along with her grandmother, Glenda Vickers - sped from Abilene to the Dallas hospital Tuesday night
"Jake stuck his finger in her hand, and she squeezed it," she said. "And then when I would talk to her and tell her grandma is there, she would move."
"She was beautiful, she was smart. She could count way up there," Vickers said. "She could say her ABC's, she could sing songs."
Jake Lively said Logan's mother told him the girl hurt herself falling at the playground. But he had doubts.
"I knew he was watching them. And I knew that when she called me, that it wasn't no playground accident," he said. "Something else happened that they weren't telling me."
Bankston said the investigation is ongoing.
"Testing will be done on the bruising and the injuries themselves, which will take several weeks," he said. "And hopefully that will give us a better idea of what was used to cause those injuries."
Authorities are holding Herman without bond at the Fannin County jail.