It was unknown whether high winds during the day contributed to the accident.
Jones, 34, was catapulted out of a cherry picker as he worked with four other crew members to remove the last of five old transformers from utility lines at the Hagerstown Fairgrounds, Zimmerman said.
According to preliminary findings, Jones was using an extended winch attached to his bucket to try to remove the transformer. The transformer still had a bolt linking it to the pole.
Zimmerman said the stress on the winch made it snap, causing the bucket to bounce down and then fly up, catapulting Jones out, Zimmerman said.
As he fell, Jones struck his head on the back of the city's bucket truck, he said.
Jones, 34, of 120 Summer St., died of brain damage resulting from multiple skull fractures, according to State Medical Examiner Stephen Radentz in Baltimore.
Lineworkers are trained to wear the safety strap around the waist while in the bucket, said Eric Marburger, the city's personnel manager. As far as city officials know, the strap was not broken.
State investigators interviewed the four other crew members at City Light's West Baltimore Street facility on Friday, Zimmerman said. They photographed the accident scene Thursday.
Investigators plan to talk to the manufacturer of the bucket truck and review the truck's maintenance records.
Zimmerman said he wasn't aware of any problems with the truck, which last underwent a routine maintenance check on Dec. 3.
After Jones fell, his co-workers didn't wait for an ambulance, but instead rushed him to Washington County Hospital themselves in a city pickup truck.
They had to make a judgment call, Marburger said. He said that hospital officials, noting the seriousness of Jones' injuries, agreed the crew had made the right decision.
Jones died at the hospital about 90 minutes after the fall.
Grief counseling was provided at the hospital for crew members. Zimmerman said the city will host a reception for family and friends at Zion Reformed Church on North Potomac Street following Jones' burial Monday.