Jessica M. Redmon-Beckstead, 17, and her boyfriend, Jason Decoste, were riding Metro bus #358 during the evening rush hour.
The video shows five female suspects board the bus, and one of the teens takes an MP3 player from Redmon-Beckstead. When she approaches them to get it back, several of the teens attack her and her boyfriend.
Several punches and kicks were thrown, and Redmon-Beckstead was cut on the eye. She was not hit in the stomach and her baby was not injured. Decoste suffered bruises.
Police say during the attack, three of the suspects stole items from the boyfriend's pockets.
Passengers yelled to the bus driver to pull over, which he finally did.
"Frankly, I think people were just so shocked that it took them a while to process what was happening," said King County Sheriff Sue Rahr.
Rahr urged anyone who sees a disturbance on a bus to call 911 immediately.
The bus driver pulled over and alerted police. Officers arrested three of the girls within 30 minutes of the attack, and the remaining two suspects a few days later. Police say the girls have records with law enforcement.
Four of the girls were charged with second degree robbery. The oldest girl, Ayana Sharee Cain, 19, was charged with second degree assault.
"I was shocked at how vicious it was and how unprovoked it was," said Sheriff Rahr. "I don't think there's a single person who didn't register some level of shock because it was so vicious and it was so completely unnecessary."
Several passengers who regularly ride #358 told KING 5 News the beating doesn't surprise them, and they've seen it before.
"I was attacked on this route one time by a bunch of young kids," one person told KING 5 News.
An additional 250 cameras are being added to Metro's bus fleet. Police patrols are also being increased on route #358.
All the suspects had their school-issued bus passes taken away, and are banned from riding a Metro bus for one year.