Police are investigating a brawl after graduation ceremonies at William Floyd High School that was captured on video and posted on social media.

Suffolk County police said they responded about noon Sunday to a report of a fight at the Mastic Beach school, but the crowd had dispersed by the time officers arrived and no arrests were made.

The three-minute video posted on Facebook begins with two people on grassy ground, fighting.

Several people are seen trying to separate them and keep others away, and one woman runs in and punches at the people on the ground, then kicks at them.

One of the people on the ground, Bianca Bouchard, 21, said she was trying to break up a fight between her sister and another relative by marriage. In the video, Bouchard, who said she’s pregnant, can be seen emerging from the tussle with blood streaming down the side of her face.

She later said the injury was minor and that she didn’t seek medical attention for it, but went to the doctor to ensure her unborn child was unharmed.

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“I’m fine now but my head is really sore,” she said Monday night.

Despite the injury, Bouchard said she is not hoping the person who struck her is charged with a crime. “I don’t plan on pressing charges,” Bouchard said. “I don’t want to ruin anyone’s graduation memories more than I already did.”

Bouchard said she does not know the name of the person who struck her, but only that she is a graduate of the school.

After the scuffle involving Bouchard, the video shows the crowd then moving toward a parking lot, where there is another fight. At that point, school security personnel move in and separate people.

There were a few people in caps and gowns on the video and none of them appeared to be involved in the violence.

Seventh Precinct crime section officers are trying to identify the people in the video and talk to them, police said.

The school district posted a statement on its website about the brawl.

“We are disappointed by the actions of those who instigated the event on what was a joyous occasion for our students and their families,” the statement said.

“As a school and community, we won’t let the small number of spectators involved in inappropriate behavior overshadow the accomplishments of so many wonderful students and their families celebrating graduation day,” it read.

“Many of our students are headed to the finest colleges and universities in the world, will serve honorably in the armed forces or begin the initial steps of their careers after learning valuable skills in one of our career and technical education academies. Graduation day is about them,” the statement said.

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The family of Keenen King, 19, of Shirley, a senior at the school who was killed in a dirt bike crash Thursday, attended the ceremony and accepted his diploma, school officials said.

Police ask anyone with information to call Crime Stoppers at 800-220-TIPS.

With Ellen Yan