West Palm Beach police say the victim was identified as a 79-year-old woman from Palm Beach County, but would not release her name due to Marsy's Law, WPTV reports.
The accident happened just after 1 p.m. Sunday on the Royal Park Bridge, when the woman was walking her bicycle westbound from Palm Beach toward West Palm Beach.
Police said she was within 10 feet of the barrier arms that halt traffic when the drawbridge began to open.
"The woman tried to hang on. There was a bystander nearby who tried to help her, but tragically she fell five or six stories below where she died landing on concrete," West Palm Beach police spokesman Mike Jachles said.
Police said a man on a skateboard desperately tried to rescue her and help her hang onto the bridge.
"Despite those efforts, the woman was not able to hold on and she fell to the concrete landing below to her death," Jachles said. "From where the victim was to where she landed was approximately five to six stories or 50 to 60 feet."
According to WPTV, West Palm Beach Fire Rescue's Technical Rescue Team had to use ropes and harnesses to lower firefighters beneath the bridge's surface to reach the victim, who landed on a concrete surface where the mechanical components of the drawbridge are located.
Police described the bridge tender as "distraught," the Palm Beach Post reported. Why the bridge tender began raising the span before it was cleared of pedestrians will be a focus of the investigation, Jachles said.
"That bridge tender has certain safety protocols to follow, specific safety protocols," Jachles said. "That includes lowering of the gates for the vehicles, lowering of the gates for the pedestrians, and making several visual confirmations that there is nobody at either of the spans or past those gates."
The bridge is equipped with bells and whistles to alert drivers and pedestrians that the drawbridge is going up.
The Florida Department of Transportation maintains the bridge, but the bridge tenders are staffed by a private state contractor.
Police are planning to review surveillance video from the bridge as part of their investigation.
The bridge was closed for more than six hours after the accident.
The Associated Press contributed to this report.