WATERVLIET — The city high school's graduation ceremony ended Thursday night with a brawl that required four police agencies before it was under control, Watervliet police said Friday. One officer was injured and two teens arrested.

Watervliet Police Chief Mark Spain said police "have no idea" what sparked the fight.

Investigators are reviewing surveillance footage from before the brawl in the hopes they'll hear "chatter between groups," he said.

Someone called 911 at 8:34 p.m. to report a "large fight" outside Watervliet High School, Spain said. The school's graduation ceremony had just ended.

Officers arrived to find about 30 people brawling in the parking lot and immediately called for backup, Spain said. Officers from Green Island, Colonie and Menands police departments responded.

A Watervliet police officer was injured in the fray and taken to Samaritan Hospital, where he was treated and released, Spain said.

The patrol officer's hand is swollen but likely not broken, the chief said. No other injuries were reported.

Police arrested 18-year-0ld Damion M. Walker, a city resident, and an unidentified teen boy. Both are charged with misdemeanor obstructing governmental administration.

Superintendent Lori Caplan said that any student or adult found in violation of the district's code of conduct would face disciplinary consequences.

"Last night's incident detracts from the celebration and the accomplishments of our graduates," she said in a statement on the district website. "It is troubling and it is unfortunate that the foolish choices and actions of a few individuals created a negative and potentially dangerous atmosphere for what began as a happy and memorable occasion for our graduates and their families. We are profoundly disappointed in the behavior exhibited last night by the students and adults involved in the altercation. We do not condone or tolerate violence of any kind in our schools."

The investigation is ongoing and more arrests are possible, Spain said.