The girl — Dajerria Becton, 15 — was detained and later released to her parents, police officials said. A lawyer for Ms. Becton told reporters that Corporal Casebolt’s stressful encounters earlier in the day were not a defense for his actions, which she described as “inappropriate, excessive and without cause.”

“Stress comes into our lives as attorneys, as reporters,” said the lawyer, Hannah Stroud. “So using that as an excuse, or what sounded like a defense, it didn’t bother me, but I wasn’t sure if it perhaps belittled the apology a little.”

Ms. Stroud said she was exploring legal options, including filing a federal civil rights lawsuit alleging excessive force. She said that Ms. Becton was an invited guest at the pool party, and that after she was asked to leave, she complied. “But when she asked for her bag so that she could call for her aunt, who is her legal guardian, she was pushed to the ground, grabbed by her head, and her face was shoved into the ground,” Ms. Stroud said.

Ms. Bishkin said the video depicted “a small part of Eric’s actions that day.” When the report of a disturbance at the pool first came over the radio, Ms. Bishkin said he was reluctant to respond “to a simple trespassing call given what he had just been through.” Once on the scene, she said, he did not target minorities, detaining a white female not shown in the video. But she added that his efforts to gather information were hampered by some teenagers who were instructing others to defy the police.

On Tuesday, the chief of the McKinney Police Department, Greg Conley, said that Corporal Casebolt would keep his pension and benefits after his resignation, but that he remained under investigation. The police department is investigating whether to bring criminal charges against him.

Corporal Casebolt, a 10-year veteran of the department who was once named its Officer of the Year, was not at the news conference. Ms. Bishkin and leaders of the McKinney chapter of the Fraternal Order of Police said that he had received numerous death threats and that those threats had forced him and his family from their home.