Irwin ``Buzz'' Gross has 36 years on the department, but it was a freak accident that left him battling head injuries at Beth Israel-Deaconess Medical Center.
``We're really at a loss as to what happened,'' Brookline fire Chief Peter Skerry said at a news conference.
Gross, a 58-year-old Natick resident, was riding in Engine 3 when it left its Washington Street station shortly after 9 a.m., investigators said.
He was sitting in the seat behind the driver.
As the engine made a right-hand turn toward Regents Circle, Gross apparently was thrown to the ground.
``At some point, he lost his balance, or whatever, and toppled from the fire truck,'' Skerry said.
Fellow firefighters ``all of a sudden realized that a firefighter wasn't where he was supposed to be,'' Brookline police Capt. John O'Leary said. ``They didn't know how he fell. It's still under investigtion.''
Investigators removed Engine 3 from service so it could be towed to a secure lot pending examination, O'Leary said.
Fellow firefighters said Gross was wearing his helmet when he fell.
What was less clear was whether he was wearing a seat belt.
``There are belts available,'' O'Leary said.
Investigators also want to determine whether there was something wrong with a door that should have prevented Gross from falling out of the truck.
Gross is married and the father of two adult children.
Reprinted with permission of the Boston Herald.