A white policeman seen in a cellphone video tossing a 14-year-old, bathing suit-clad black girl to the ground and burying his knees in her back resigned Tuesday from the McKinney police force.

Former McKinney Police Cpl. Eric Casebolt had been placed on administrative leave as the department undertook an investigation of his actions during the June 5 disturbance in the city about 30 miles north of Dallas.

The incident raises new questions about racial bias in U.S. policing and excessive use of force by police — in this instance, against children.

In the video, Mr. Casebolt is seen shouting obscenities at black youths in a multiracial crowd, shoving a black teenage girl, briefly pointing his gun at black youths and violently slamming Dajerria Becton, 14, to the ground, pinning his knee in her back. At one point, he had both knees on her back.

The seven-minute video posted on YouTube had been viewed 9 million times as of Tuesday morning. It shows officers responding to the incident, which police said started when scores of young people attended a party with a disc jockey at a community pool and refused requests to leave.

According to neighbors, a woman who lives in the community reserved the pool for a party, said Benét Embry, a black local radio personality who witnessed the incident.

The homeowners association limits the number of guests each homeowner may have at the pool to two. But about 130 people, mostly kids, showed up. At one point, several kids began jumping over the fence to get into the pool area and were causing a disturbance, Mr. Embry said. A couple of fights broke out.

“This was a teenage party that got out of hand,” Mr. Embry said.

Police said some of the young people did not live in the area and did not have permission to be at the pool.

A white teenager captured the events on the video that went viral. In the video, Ms. Becton repeatedly cries out, “Call my momma!” as Mr. Casebolt pins her to the ground, only moments after drawing his handgun on other teens.

“On your face!” he yelled at the girl, amid screaming from a crowd of onlookers.

After Mr. Casebolt’s resignation, McKinney Police Chief Greg Conley called Mr. Casebolt’s actions “indefensible.”

Chief Conley said at a news conference Tuesday that Mr. Casebolt, 41, resigned “on his own will” while under investigation and will get to keep his pension and benefits.

“Our policies, our training and our practice do not support his actions,” Chief Conley said. “He came into the call out of control and, as the video shows, was out of control during the incident.”

“I had 12 officers on the scene and 11 of them performed according to their training,” the chief added.

He said it will take some time to determine whether Mr. Casebolt should be criminally charged.

Civil rights leaders in McKinney are calling for a federal investigation.

In an interview with local media, Ms. Becton said the officer twisted her arm and grabbed her by the hair.

“Him getting fired isn’t enough,” she said.

Police officials saw the video for the first time Saturday afternoon, Chief Conley added.

Hundreds of people rallied in McKinney on Monday night calling for Mr. Casebolt’s firing.

Chief Conley said when Mr. Casebolt resigned Tuesday, he didn’t apologize or make any statement. “It was just a simple resignation: ‘I resign,’ ” the chief said.

On Wednesday, Jane Bishkin, an attorney for Mr. Casebolt, sought to explain his actions by noting he had responded to two suicide calls earlier in the day. They had taken an “emotional toll” on him, she said.

Ms. Bishkin says Mr. Casebolt and his family have been forced to leave their home because of death threats following the video’s release.

At the start of the video shown on most media outlets, one officer cordially tells some teens: “Don’t take off running as soon as cops get here.”

Several wrote on the McKinney Police Facebook page that the youths antagonized police and should have obeyed when police told them to sit down and keep quiet. A few area residents told media the case was about unruly teens and not about race.

McKinney has about 150,000 people, according to the U.S. Census Bureau. African-Americans make up 10.5 percent of the population and white people about 75 percent.