Authorities are investigating why a crane and part of a parking garage roof collapsed late Saturday, killing the crane operator who plummeted nine stories along with heavy debris. The accident happened at the Government Center, which is undergoing a $1.5 billion redevelopment project. One passerby was also injured but is expected to survive. “This is a horrible tragedy and my heart goes out to the family and loved ones of the worker here tonight,” Boston Mayor Michelle Wu said Saturday. “This is a very complex site and we know that the workers who take on the risks of this should never have to worry about, their family should never have to worry about if they're going home at night.”