"Columbo" was a popular US television detective series starring Peter Falk. What was Lt. Columbo's first name?

Columbo was an American television series (1968-2003) starring Peter Falk as a homicide detective with the Los Angeles Police Department. Lieutenant Columbo's first name is never explicitly mentioned during the series. Even the opening credits just simply read, "Peter Falk as Columbo".

Anyway, the attentive watchers could mention the name "Frank" on Columbo's police ID, which was clearly shown in some episodes of the show. For example, in the 1971 episode "Dead Weight" Columbo shows his badge to General Hollister, and the signature is seen for a short while. Another episode, named "Death Hits The Jackpot" (1991), can also let the watchers figure out Columbo's first name. The main character shows how shiny his badge to Rip Torn's character. When Columbo holds his badge up, the name "Frank" is clearly typed there.

Not only Columbo's first name is almost never seen and pronounced in the show. According to the plot, Lieutenant has a wife and he is often talking to her on the phone, but the watchers are never to see Mrs. Columbo in the show.