A Pennsylvania police sergeant is under investigation after a video showed him hitting a woman in the face with a stun gun during a St. Patrick’s Day street brawl.

A 10-second clip posted to Twitter on Saturday shows an unidentified Chester police sergeant striking Dominique Difiore, 20, of Brookhaven, with what appears to be a Taser as cops tried to control several people who were fighting in the street.

Police Chief James Nolan said Difiore struck the sergeant first but that cannot be seen on the footage.

“A partial video of the event is circulating on various social media platforms,” Nolan said in a statement. “The incident, the video, and level of force used in connection with the event are all currently under investigation.”

The fight took place in the 800 block of East 16th Street at about 5 p.m. Saturday. Officers responding to the scene were told a “riot” had broken out after several people were turned away from a nearby house party on East 15th Street, Nolan said.

Jaylene Westfall, 19, of Levittown, assaulted one of the residents at the home after being turned away, prompting the partygoers and residents to lock themselves inside the home. But Westfall and other suspects managed to kick in the home’s front door and assaulted several people once inside, Nolan said.

In all, four people were arrested and charged with aggravated assault and alcohol-related offenses. In addition to Westfall and Difiore, they were identified as Shawn Connelly, of Philadelphia, and Tess Herman, 20, of Springfield.

Investigators are still looking into whether other people will be charged in connection with the incident.

Sources told CBS Philly that the fight happened during a Widener University pub crawl. But university officials told the station that it was not a “university-sanctioned event” and said the four people arrested were not students at the school.

“It was just very hectic,” said Zachary Whalen, a senior at Widener. “I would say probably 30, 40 people were just running around here, beating the crap out of each other.”

Several witnesses said they saw the officer use his Taser on Difiore once she fell to the ground.

“I was shocked,” witness Latine Bethea told CBS Philly. “I was really shocked. I couldn’t believe a man hit a female like that.”

One man who lives nearby said he thought the officer’s blow to Difiore was a clear example of excessive police force.

“I think it was ridiculous to do something like that, no matter how frustrated you get,” neighbor Matt Pfaff said. “They’re trained to do better.”

A message seeking comment from Chester police early Monday was not immediately returned.