“They literally were filming in my dad’s office,” Ms. Toschi-Chambers said. “My dad took off his jacket, and Steve McQueen said, ‘What is that?’ And my dad said, ‘That’s my holster.’ And Steve McQueen told the director, ‘I want one of those.’ ”

Clint Eastwood also drew on Mr. Toschi for his portrayal of the title character in “Dirty Harry,” Don Siegel’s influential 1971 movie about a San Francisco police inspector, Harry Callahan, who hunts a psychopathic killer. Mr. Toschi, though, was bothered by Callahan’s penchant for administering his own brand of justice. He is said to have walked out of a screening of the movie, which was released when the Zodiac investigation was in full swing.

“He couldn’t take it,” Mr. Ruffalo, who spent time with Mr. Toschi preparing for his “Zodiac” role, said in a 2007 interview with the website Collider. “It was so simplified.”

Mr. Toschi’s daughter said he always thought that a suspect named Arthur Leigh Allen, who died in 1992, was Zodiac. But, unlike Harry Callahan, he and his fellow officers were bound by the evidence.

“If you get into who these cops were,” Mr. Ruffalo said in another 2007 interview, “you realize how they have to take their hunches, their personal beliefs, out of it. Dave Toschi said to me, ‘As soon as that guy walked in the door, I knew it was him.’ He was sure he had him, but he never had a solid piece of evidence. So he had to keep investigating every other lead.”

In a 1978 interview, Mr. Toschi estimated that he had talked to 5,000 people during the Zodiac investigation. His letter-writing misstep not only took him out of the investigation that year, but also brought a more serious accusation — that he might have written a letter from around that time that claimed to be from the Zodiac killer. He vigorously denied that speculation.

Other letters claiming to be from Zodiac turn up periodically, most of them hoaxes, and new theories and even confessions surface from time to time. The case has inspired numerous books and episodes of true-crime television shows, including a five-part series on the History channel that concluded in December.