The attorney for a Gloucester Township cop who struck a 13-year-old girl twice in the face in 2018 told jurors during opening arguments Tuesday that while the video of the incident may appear “offensive,” Officer John Flinn used only as much force as necessary to get the “out-of-control” girl into cuffs.

But the other officer who was holding the girl’s right arm and leg during the cuffing told jurors that while the girl had been screaming and resisting staff at the group home moments before, she was calm and quiet when Flinn delivered the slaps to the right side of her face.

“She was just lying there,” Paul Bertini, now a Gloucester Township detective, said twice in court Tuesday.

Flinn, 29, of Sicklerville, is facing charges of endangering the welfare of a child and two counts of official misconduct in connection with the March 8, 2018 incident at Twin Oaks, a group home Bertini said houses girls with mental health issues.

Flinn was suspended after the incident and later charged. The release of body camera footage of the incident sparked outrage.

Suspended Gloucester Township Officer John Flinn, right, stands in Superior Court in Camden with his attorney, Louis Barbone, left, in a 2018 photo.Rebecca Everett | For NJ.com

His trial began Tuesday in Superior Court in Camden County, with opening statements from Assistant Prosecutor Angela Seixas. Seixas told jurors that the girl was compliant when Flinn smacked her twice on the face, pushed her head down on the pile of junk she was lying on, and put his knee on her back.

“It was too much,” she said.

Flinn’s attorney, Louis Barbone, argued that the body camera video shows the girl is moving her right arm away from Flinn’s attempts to cuff her, and he acted to quickly get her into handcuffs given the chaotic scene at the group home.

Bertini, whose body camera footage was shown to jurors twice Tuesday, testified that police were called to Twin Oaks around 6:30 p.m. that day for a report of juveniles fighting each other and “using objects as weapons.”

Inside the home, he said he followed the sounds of “commotion and yelling” and found two staff members struggling with a 13-year-old girl in an office and storage space. The video shows staff holding both her arms as she tries to pull away.

Barbone told jurors that the video shows the girl’s strength.

"Put the idea of age out of your mind. It’s irrelevant. What’s relevant is power. What’s relevant is force,” he said.

Bertini testified that after the girl ignored his orders to calm down he took her to the ground to stop her leaving and in case she had something that could be used as a weapon. She was lying there with Bertini holding her right arm and leg when Flinn came in and took control of her left side, Bertini said.

Barbone said the girl moved her right arm in a way to prevent him from getting the second cuff on her, and that was why Flinn struck her.

Bertini initially testified that he had control of the right arm, did not feel the need to use more force and did not know why Flinn struck her and pushed her head. He also said he did not know what Flinn meant when he yelled for the girl to "stop playing these f---ing games!”

But on cross examination, Bertini admitted that the video did show the girl moving her right arm.

“Isn’t that resisting?” Barbone asked him.

“You could say that,” Bertini said.

Seixas told jurors that police are only allowed to use the level of force that a person would find was “reasonable and necessary,” given the circumstances, to arrest or detain someone.

Barbone argued that the force was the only thing that allowed Flinn to cuff the girl — something he was anxious to do because of the reports of other fights going on in the group home at the same time.

He also told jurors he knew they would have a “visceral” reaction to the video.

“When you first look at that, it’s offensive. You say, 'Why did he have to smack her like that?’ That’s what I said. And that’s the human reaction,” he said. But he told jurors they had to step back and consider the chaotic situation, the need to get the girl under control and what he says was her resistance to the officers holding her down.

Judge Edward J. McBride Jr. is presiding over the trial. Seixas said in court that she plans to call witnesses until sometime Thursday. Both sides plan to call expert witnesses who will testify about what constitutes reasonable force.

Rebecca Everett may be reached at reverett@njadvancemedia.com. Follow her on Twitter @rebeccajeverett. Find NJ.com on Facebook.

Have a tip? Tell us. nj.com/tips