“It was like the devil was in there.”

That’s how the dad of a 16-year-old Philadelphia high school student who was brutally beaten, including with chairs, described what happened Tuesday to his son. And now, five of the attackers involved have been arrested and charged.

The beating occurred inside the Chester High School library. One of the attackers took video of it and posted it on Facebook. But it wasn’t the school that alerted police — a fellow student did. The school was unaware.

When the local CBS station in Philadelphia asked the school how it went undetected, they said they were unsure.

“We don’t know that yet but we’re investigating and the proper action will be taken when we know,” an official said.

“The kids are really taking over the school there,” the dad — who spoke to the local station and asked to remain anonymous — said. “I think security really needs to be stepped up in there.”

The five attackers, who are all being charged as adults, have been hit with simple assault, aggravated assault, conspiracy, robbery, recklessly endangering, terroristic threats and possession of instrument of a crime.

But police say they are not done with their investigation and more people could be charged.

“We’re going to treat each incident individually,” Chest Mayor John Lindner said. “We’re going to investigate it thoroughly and obviously we don’t like having our school district painted in this color or our city in this color.”

It’s still unclear what prompted the incident, but a full video posted on the local NBC Philadelphia affiliate appears to show it was random. That outlet has more on how it unfolded:

On Wednesday, police arrested five teens who they say were involved in the attack. Omar Davis, 19, Citerrah Gomillon, 16, Jordan Morales, 16, Clayton Newman, 18, and Edward Stanley, 18, were all charged as adults. All five teens are charged with simple assault, aggravated assault, terroristic threats and other related offenses. Each teen is behind held on $100,000 bail.

Sources close to the investigation tell NBC10′s Nefertiti Jaquez that Davis threw the first punch, Morales picked up a chair and slammed the victim’s back, Newman repeatedly kicked and punched him before taking his cell phone while Gomillon kicked the victim after he was already knocked unconscious.

The source also says that Stanley is the teen who recorded the fight and posted the video on Facebook. While Stanley isn’t seen attacking the victim, investigators say he did nothing to stop the fight and could be heard laughing throughout.

You can watch the full video here.