Mildred Hetmanski, 83, of Crete, was one of three people hit when a branch fell from one of the park's oldest trees at about 1 p.m. June 8, said Robert R. McCormick Foundation spokesperson Phil Zepeda.
Hetmanski and another individual were transported to Central DuPage Hospital, while a third declined treatment.
The Examiner's Office could not confirm the time of death, as reports were preliminary, but an autopsy was carried out Wednesday. The death was ruled an accident and additional details may be released Friday.
"We offer our condolences and our prayers to Mrs. Hetmanski's family," said Foundation CEO and President David Hiller in a statement.
The branch, which measured 80 to 100 feet long, was a part of a historic Bur Oak tree near the east side of the McCormick Museum. The tree is more than 300 years old, Zepeda said, and is at least 7 feet in diameter.
Zepeda said nothing comparable had ever happened on park grounds before, and the falling branch is being treated as an isolated incident. He said the park has a staff of horticulturalists who regularly monitor the safety and health of its trees. The tree from which the branch fell is currently roped off to ensure safety.