SYLMAR, LOS ANGELES (KABC) -- Disturbing video surfaced from inside one of Los Angeles County's juvenile halls, which shows four probation officers beating up a teenager in their care as a supervisor looks on.The incident, which escalated in a holding room at L.A. County's Barry J. Nidorf Juvenile Hall in Sylmar, was under investigation.The video is silent. We don't know what words are exchanged with officers who enter, but in seconds, we do see the violent outcome.Celeste Fremon, editor of WitnessLA , said she obtained the four-minute video from sources inside the probation department.For years, L.A. County juvenile facilities had been monitored by the Department of Justice because of overcrowding and excessive force. Reforms were implemented and in 2009, investigators signed off.Whistleblowers inside the system were now worried again."There is still a culture of brutality against juveniles, and they are very upset about it," Freman said.On Friday, the probation chief, Cal Remington, responded to the incident saying the department takes mistreatment of juveniles very seriously."We hold our staff to a very high standard. We have zero tolerance for mistreating the juveniles in our care. And while I can't really comment on an ongoing investigation, this matter is being thoroughly investigated, and we take matters of this nature very, very seriously," he stated.What triggered the takedown? Moments earlier, the juvenile is seen throwing something bulky at the door, then a milk carton. But alone inside his cell, he does not appear to be a threat to anyone."There is no reason for this kind of force - ever, under any circumstances, even if you have a riot," Fremon added.The L.A. County Probation Department turned over the case to the District Attorney's Office. Prosecutors in the Justice System Integrity Division will determine whether criminal charges will be filed.