In the movie Zodiac (2007), from the initial murders, we see that Mike Mageau is shot by the killer but somehow survives in 1969. After this incident several other murders take place and at one point of time they are suspecting Arthur Leigh Allen and bring him for questioning but he's let go because of handwriting mismatch. Much later around 1983, they bring in Mike to identify the killer and among six pictures of possible killers, he recognizes Arthur Leigh. Why wasn't Mike identified for questioning much before? Why was so much time wasted before he was brought in?
It has been addressed in the movie itself.
#1 Following conversation happens between Detective Armstrong and Vallejo Police department after the Bus Threat goes on TV.
ARMSTRONG: We're playing catch up here. I'd like to set up a meeting with your survivor, Michael Mageau-
DAYMULANAX: Can't. Skipped town.
ARMSTRONG: The only guy who's seen Zodiac without a mask is missing?
MULANAX: When he was still in the hospital we could show him line ups every day. Soon as he got out he left. Don't think he wants to have anything to do with this.
#2 Graysmith has the same question for Detective Toschi when they meet outside his office at night.
GRAYSMITH: Thanks for meeting me, Inspector.
TOSCHI: We're not meeting. I gotta be back in five minutes-
GRAYSMITH: Did anyone ever show Mike Mageau suspect photos?
GRAYSMITH: He's the only surviving victim who saw Zodiac without a mask.
TOSCHI: No, why are you asking me? Darlene and Mageau are Vallejo's investigation, Paul Stine is mine, you got four minutes now-
GRAYSMITH: Darlene Ferrin was being followed.
GRAYSMITH (CONT'D): I know Mulanax says they already found this guy, a George Waters. But he also says Darlene had lots of "admirers" who would come by the restaurant where she worked. When Darlene and her husband moved into their new house, they threw a painting party-
TOSCHI: What's a painting party?
GRAYSMITH: It's a party where people come over and help you paint.
TOSCHI: That sounds like a terrible party.
GRAYSMITH: But someone who was not Waters showed up that night and Darlene was really scared of him.
TOSCHI: You think Zodiac knew Darlene.
GRAYSMITH: And if Zodiac knew her, maybe Mageau knew Zodiac.
TOSCHI: We’ll never know. Mageau's gone. In the wind. If you want to connect Zodiac to Darlene, you're gonna have to find another way.
Refer to following timed part.