(CNN) -- An 80-year-old woman who died after being found unconscious in a restroom at an eastern Georgia McDonald's was killed by a lethal dose of carbon dioxide, authorities said Wednesday.
An investigation determined that a "bleed line" on the tank used to carbonate beverages in the McDonald's drink dispenser was "improperly disconnected within the wall cavity," Pooler, Georgia, Police Chief Mark Revenew said.
This caused carbon dioxide to build up in the restroom, which was in close proximity to the tank, "in a potentially lethal concentration," he said.
Anne Felton, of Ponte Vedra, Florida, was one of two people found in the ladies' restroom September 7. Authorities arriving on the scene were also stricken by "an odor," Revenew said at the time.
A total of nine people, including three firefighters, were transported to a local hospital, where Felton died.
No one else exposed during the incident should suffer any long-term effects, Revenew said Wednesday. Simmons said all three firefighters have recovered and returned to work.
At certain levels, Pooler Fire Chief Wade Simmons told reporters Wednesday, carbon dioxide can "render you unconscious in a very short period of time."
Authorities are still investigating how the incident occurred, Revenew said, adding that it involved "more than one error, I believe." The franchise's owners, he said, are "grief-stricken" and have been very cooperative. The findings so far are considered preliminary, he said.
Investigators did not discover the issue until Tuesday night, after receiving a call from a police department in Phoenix, Arizona, advising them of a similar incident there, Simmons said. That call "sped us up and moved us in the right direction," he said.
The restaurant re-opened after the incident when it was deemed safe. Authorities said it was closed Tuesday night for testing and the carbon dioxide issue was found. Revenew said the wall cavity had to be opened during the testing.
Pooler is about 10 miles west of Savannah, Georgia.