Larry Johnson, a former top executive at Alcor Life Extension Foundation, revealed the incident in a new book. He told Tuesday's Boston Herald that he feared his life would be in danger if he went to authorities or the former Red Sox slugger's family with news of the accident.
"I was too scared for my life to say anything to anyone publicly until I was out of there," Johnson, a former emergency medical technician, told the newspaper. "It was enough to shock a veteran paramedic. I just kind of gasped."
In his book, "Frozen: My Journey Into the World of Cryonics, Deception and Death," Johnson alleges that another Alcor employee whacked Williams' preserved head with a monkey wrench while trying to free it from atop a tuna can.
The company has denied the allegations. Williams body was frozen in 2002.
Johnson said he cried after the incident, telling the Herald, "I was scared to death. I knew that this was so outlandish, so unbelievable, nobody was going to believe me."