CHICAGO, Illinois (CNN) -- Chicago police asked the public for help Tuesday in finding three more people believed connected to last week's videotaped fatal beating of 16-year-old Derrion Albert.
Derrion Albert, 16, was beaten to death last week. His death was captured on video.
"We need the public's help, your help," Chicago Police Superintendent Jody Weis told reporters.
With four teens already arrested and charged with first-degree murder in the killing, "we are continuing to look for three additional people in connection with Derrion's murder," Weis said.
Weis said he has asked the U.S. Secret Service to try to enhance the video of the beating so that others can be identified.
Weis pleaded with anyone who may have information not to withhold it.
"The culture of 'no-snitch' is unacceptable," he said. "A young man with a promising future lost his life to senseless violence, yet few have come forward."
Authorities are considering charging people who participated in the fight but did not come into contact with Albert, he said.
Prosecutors said Albert was a bystander who ended up in the middle of a street fight Thursday between two factions of students from Christian Fenger Academy High School, on Chicago's South Side.
A video recorded by a witness, which has been broadcast widely, showed the attack unfolding. A local TV station that received the video turned it over to police. Watch graphic video of the attack »
When school let out at 2:50 p.m. Thursday, Albert was nearly six blocks away -- on his way to a bus stop -- when two groups of students converged on the street, Tandra Simonton, spokeswoman for the Cook County state's attorney, said Monday.
The factions, one that lived near the Altgeld Gardens housing development and one in an area known as "The Ville," began fighting after a shooting earlier that day that police called gang-related.
Albert was approached by a person later identified as 16-year-old Eric Carson and an unidentified person, both members of the "The Ville" faction, Simonton said. Carson struck Albert in the head with a piece of a wooden railroad tie, and the second person punched him in the face, Simonton said.
After being knocked unconscious briefly, Albert regained consciousness and tried to move from the fight but was then attacked by a second group, Simonton said.
That group, composed of five members from the opposing faction, then took their shots at Albert, Simonton said.
He was struck in the head by Eugene Riley, 18, with the piece of railroad tie, a rectangular piece of wood used as a base for railroad tracks, Simonton said.
Once Albert was on the ground again, 19-year-old Silvanus Shannon was seen "stomping on his head repeatedly," Simonton said.
Albert was taken to Roseland Community Hospital and then to Advocate Christ Hospital and Medical Center, where he was pronounced dead.
Simonton said the first three suspects arrested in Albert's death -- Shannon, Carson and Riley -- were seen on the videotape attacking Albert and were charged with first-degree murder and held without bail. All three were charged as adults, she said.
A fourth person, Eugene Bailey, 17, was arrested Monday and faces the same charge, authorities said.
"We have the four most egregious offenders," Weis said.
Weis said Chicago police will beef up their presence around the school before and after classes.
CNN's Mallory Simon contributed to this report.