In the TV series "Ironside", how did Ironside become paralyzed, leaving him in a wheelchair?

"Ironside" is an American TV crime drama that aired on NBC over 8 seasons from 1967 to 1975. The show starred Raymond Burr as Robert T. Ironside (usually addressed by the title "Chief Ironside"), a consultant for the San Francisco, CA police department (formerly Chief of Detectives), who was paralyzed from the waist down after being shot while on vacation. The storyline debuted on March 28, 1967, in a TV movie entitled "Ironside". When the series was broadcast in the UK, from late 1967 onward, it was broadcast as "A Man Called Ironside". The show earned Burr six Emmy and two Golden Globe nominations.

"Ironside" was a production of Burr's Harbor Productions Unlimited in association with Universal Television. The TV series specifically revolves around former San Francisco Police Department (SFPD) Chief of Detectives Robert T. Ironside (Burr), a veteran with more than 20 years of service. He is forced to retire after a sniper's bullet to the spine paralyzed him from the waist down, resulting in his reliance on a wheelchair. In the pilot TV movie, Ironside shows his strength of character and gets himself appointed a "special department consultant" by Police Commissioner Dennis Randall. He does this by calling a press conference and then tricking Commissioner Randall into meeting his terms for employment. In the movie, Ironside eventually solves the mystery of his ambush and facts on the shooting. He gets police detectives Ed Brown and Eve Whitfield assigned to aid him.