“The video that I have reviewed certainly raises concerns,” the chief said.

Calls to the department and to the city of Fresno, both named as defendants in the lawsuit, were not returned on Thursday. The lawsuit also names as defendants Officer Christopher Martinez and “Does 1-25.”

In a report Officer Martinez wrote on the event, which he described as “gang related,” he said that he had “grabbed” Mr. Wallace because it looked like he was not listening to orders to sit down. He wrote that Mr. Wallace took a “fighting stance,” and he feared the teenager was going to push officers over the railing.

“I punched Wallace approximately three times in the face in order to get him off me and to back him up,” Officer Martinez wrote. He said Mr. Wallace continued to resist, so he “struck” him “approximately two more times” when he was on the ground.

Another officer, R. Loza, wrote in his report that he had used his “forearm to strike” Mr. Wallace twice while trying to get control during the struggle.

Chief Dyer said at the news conference that further “appropriate action” could be taken depending on the results of the investigation, which will be based on interviews and more scrutiny of the videos.

“We do know that a struggle ensued,” Chief Dyer said. “We also know that the officer swung his fist at the individual several times.”

It was “difficult” to see how many blows made contact, he said. “But we do know that he was struck at least once with one of those blows,” he said, referring to Mr. Wallace.