McKinney Police Chief Greg Conley said at a news conference Sunday that the incident began when officers responded Friday to a report of a disturbance involving a group of juveniles at a neighborhood pool.
The police department said in an earlier statement that the young people did not live in the area or have permission to be there.
Tap to see photo of protest in McKinney
When officers arrived, residents and private security pointed out the juveniles, who were "creating the disturbance, fighting and refusing to leave," Conley said.
As officers dispersed the crowd, the 14-year-old girl was "temporarily detained" by an officer, said Conley, who did not describe what led to her detainment.
The the seven-minute clip posted to YouTube showed the officer pulling the bikini-clad girl to the ground then appearing to use his knees to pin her facedown. She can be heard screaming, "I want to call my mom. He hit me."
The officer can also be seen pointing his gun at other teens and cursing.
KDFW-TV spoke to the teen in the incident who identified her as Dajerria Becton.
"He told me to keep walking and I kept walking and I guess he thought we were saying rude stuff to him," Becton said. "He grabbed me, twisted my arm on my back and shoved me in the grass and started pulling the back of my braids. I was telling him to get off me because my back was hurting bad."
"He was just aggressive for no reason at all. It was horrible," Tatyana Rhodes told ABC affiliate WFAA-TV.
Rhodes, along with her mother and sister, had organized the pool party as part of an end-of-school celebration, BuzzFeed News reports. Teens at the party says the incident began when adults at the pool "told the black children to leave the area and return to 'Section 8 (public) housing.'"
The police statement said the video "has raised concerns that are being investigated" by the department.
McKinney Mayor Brian Loughmiller echoed that sentiment. During a previously scheduled city council meeting, Loughmiller said city leaders "take this matter very seriously."
"We really need to come together as a community," Loughmiller said.
The local police union in McKinney issued a statement late Monday stating that they had not yet reviewed all of the available evidence in this case. They did insist that Corporal Casebolt acted appropriately. They also said that the McKinney Police Department does not engage in racial profiling. Both sides are urging peaceful protest.
McKinney is about 35 miles northeast of Dallas.
The Associated Press and ABC News contributed to this report.