Officer Wojckiech Braszczok's lawyer filed motions, and he and his client left. The cases against the other 10 motorcyclists, who have reportedly been offered plea deals, were adjourned because the judge was out.
Prosecutors say Braszczok was caught on camera chasing an SUV and shattering its back window before other men pulled the driver out of the vehicle and beat him.
Braszczok, who has consistently covered his face to shield himself from reporters, was off duty at the time. He has pleaded not guilty, and his lawyers have said that he never came close to the SUV driver.
SUV driver Alexian Lien has not been charged even though he ran over one of the bikers, sparking the chase, and he has filed a legal notice saying he plans to sue the city over Braszczok's role.
The tension began when an SUV carrying a family on a Sunday drive crossed paths with a motorcycle rally - with at least one rider capturing events on a helmet-mounted camera and later posting video online.
Police say a motorcycle slowed in front of the Range Rover, which then bumped the bike. Motorcyclists dismounted and converged on the SUV. Lien has said he feared for himself, his wife and their toddler. He hit the gas to get away, running over and seriously injuring motorcyclist Edwin Mieses Jr., of Lawrence, Mass.
The other bikers chased after Lien, dragged him from his car on a side street and beat him, some bashing him with their motorcycle helmets, authorities said. He needed stitches to his face.
Braszczok wasn't accused of hitting Lien, but prosecutors have said he shattered the SUV's back window and did nothing to end the assault or get help. The 10-year police veteran told authorities and his union that he didn't intervene partly because he works undercover.
Crushed by the SUV, Mieses suffered broken legs and severe spine injuries. He has gone home from a hospital but still needs a wheelchair, and he most likely never will walk again.
The Associated Press contributed to this report.