Protesters marched Monday over the incident in the Dallas suburb. Outraged parents demanded the officer's firing.
At a press conference Tuesday announcing Casebolt's resignation, the city's police chief called his actions in the video indefensible.
"Our policies, our training, our practice, do not support his actions," Police Chief Greg Conley said. "He came into the call out of control, and as the video shows, was out of control during the incident."
Officials previously had said Casebolt was on administrative leave as police investigated what happened. Now that he has decided to resign, the internal affairs investigation is closed, and it's too soon to say whether he'll face charges over what happened, Conley told reporters.
"We're continuing an investigation. We are continuing looking into all the allegations that are being presented to us, and any part of a criminal investigation regarding anyone will take a matter of time for us to work through all those allegations and those people who have come forward to us to complain," he said.
Casebolt's attorney has not responded to requests for comment.
Even as protests mount, some witnesses have praised the way Casebolt handled the incident, as dramatically different accounts emerge of whether race played a role
in the situation and its aftermath.
'It's the first step'
Tatyana Rhodes, who was hosting a party at the pool Friday before police arrived, said tensions were running high after a racially charged fight broke out there. It started, she said, when two white women told a group of African-American teens they should leave and "go back to their Section 8 homes."
One of the women, she said, smacked her in the face.
But Rhodes said the police officer took things too far.
"He didn't have to use aggression," she told CNN's "Erin Burnett: Outfront."
Now, she says she's glad he has stepped down.
"I'm happy that he's resigning," she said. "I feel that everyone in McKinney will feel better that he's resigning. ... It's the first step."
White witness: It was racially motivated
Brandon Brooks, the 15-year-old white teen who shot the video, said there's no doubt race was a factor in how police responded. He said the officer was targeting black teens at the scene.
"I was one of the only white people in the area when that was happening," he told CNN affiliate KDAF
. "You can see in part of the video where he tells us to sit down, and he kinda like skips over me and tells all my African-American friends to go sit down."
Brandon said the argument that drew police to the pool party didn't even involve most of the teens Casebolt was going after.
"It was a fight between a mom and girl, which had nothing to do with all the other kids," he told KDAF.
Brandon said he was unnerved to see his friend tackled and pressed to the ground.
"I think she was 'running her mouth,' and she has freedom of speech, and that was very uncalled for him to throw her to the ground," he said.
Dajerria Becton told local station KDFW
that she obeyed Casebolt's order to leave when he grabbed her.
"He told me to keep walking," she said. "And I kept walking and then I'm guessing he thought we were saying rude stuff to him.
"He grabbed me, twisted my arm on my back and shoved me in the grass and started pulling the back of my braids," Dajerria told KDFW. "I was telling him to get off me because my back was hurting bad."
In the video, the officer places her hands behind her back and he keeps her down on the ground.
Dajerria wasn't charged, McKinney police said. She was released to her parents.
Can't accept 'that a grown man' acted that way
Jahda Bakari, 13, said she was frozen with fear when she saw Casebolt take out his gun.
"When he pulled that out, that's the only reason I did not run away," she said.
Her father, Jahi Bakari, said he hopes to get his daughter counseling later. When she came home, she was initially afraid to even tell her parents what had happened, he said. He was "mortified" to hear her describe what she saw, he added.
He said he sees only a racist cop when he watches the YouTube video.
"We will not accept the fact that a grown man, an adult officer, will handle a female like that," he said.
Black resident: This was not a racially motivated event
Benet Embry, a 43-year-old black resident, saw things differently.
Embry has lived at Craig Ranch, a planned community in McKinney, for eight years. He said it's a nice place. Racially diverse. People get along there.
He said when he was at the community pool Friday, a crowd of teenagers showed up, even though Craig Ranch's strict homeowners' association rules prohibit bringing more than two guests to the pool.
The teens huddled by the gate and shouted to let them in. Some jumped over the fence, Embry said. A security guard tried to get them to leave but was outnumbered, so the guard called police.
"Let me reiterate, the neighbors or the neighborhood did not call the police because this was an African-American party or whatever the situation is," he said. "This was not a racially motivated event -- at all. This whole thing is being blown completely out of proportion."
Embry did say he was disturbed to see the officer kneel on top of the bikini-clad girl and wave his gun at other teens.
"I may or may not agree with everything that the police officer did, but I do believe he was trying to establish order," he said.
Police chief: Department is 'held to a high standard'
Conley, the police chief, told reporters Tuesday that Casebolt was the only one of the officers who responded to the incident who had been out of line.
"I had 12 officers on the scene, and 11 of them performed according to their training. They did an excellent job," he said. "Our citizens called us to a fight in progress and general disturbance at the community pool. We responded. I do not condone the actions of those individuals who violated the rules of community, showed disrespect to the security person on scene, and to the officers who responded. However, we as a department are held to a high standard of action as we do our jobs."
Police union officials said that teens and adults were trespassing at the privately owned pool, and that there were reports of vandalism and fighting in the area when officers responded.
"The McKinney (Fraternal Order of Police) assures that this was not a racially motivated incident," the union said, "and can say without a shadow of doubt that all members of the McKinney FOP and McKinney (Police Department) do not conduct racially biased policing."
But Brandon, the white teen who shot the video, doesn't buy it.
"They're just going to discriminate against them because they're black," he said. "What if that was your kid getting slung to the ground? Would you still be talking about them in the way that you are?"