William Martin, the lead prosecutor of Chicago mass murderer Richard Speck, has died at 80, according to his family.

The Speck case launched Martin — who continued to practice law in west suburban Oak Park until he recently took ill — to prominence.

In 1966, the 24-year-old Speck fatally stabbed and strangled eight nurses in a South Side townhouse. Martin, an assistant Cook County state’s attorney and graduate of Fenwick High School, took the lead in the prosecution.

“In a way, it was the end of innocence,” Martin told the Chicago Sun-Times in 1991. “In this case, eight women asleep in a middle-class, crime-free, virtually suburban neighborhood were subject to random violence from a killer who basically came out of the night.”

Two days after the murders, Speck tried to commit suicide by cutting his wrists in a seedy flophouse in the 600 block of West Madison — then Skid Row. Police said the then-unidentified killer had a tattoo on his arm that read “Born To Raise Hell.” A Cook County Hospital surgeon recognized it when the bloodied Speck was brought in on a gurney.

Martin secured a guilty verdict, aided by the testimony of the one nurse who survived the attack, after less than an hour of jury deliberation.

Speck was sentenced to death, though the Illinois Supreme Court ultimately overturned the sentence, leaving Speck with a life sentence. He died of a heart attack in Joliet’s Stateville Prison in 1991.

According to his son, Martin was personally against the death penalty. In Speck’s case, though, he thought the sentence was fitting.

“He told me that, in that situation, he supported the death penalty,” his son, Cook County Judge Marc Martin, told the Sun-Times Saturday. “We had long conversations about it over the years, and I don’t think [the death sentence] tormented him in that case.”

William Martin would eventually co-author a book about the case titled “The Crime of the Century: Richard Speck and the Murders That Shocked a Nation.”

Though he was comfortable with Speck’s initial sentence, Marc Martin said his father “was really a liberal at heart.”

“My dad got into the practice of law because he wanted to help people,” he said. “He’d tell me about feeling guilty for prosecuting someone for stealing a loaf of bread.”

William Martin left the state’s attorney’s office in 1969 to join the faculty at Northwestern University’s Law School.

For a short period of time in the early 1970s he went into private practice. Until his death, he was a private practitioner specializing in attorney ethics and criminal law.

William Martin lived in Oak Park for most of his life but was a resident of Riverside at the time of his death.

Marc Martin — one of the attorneys who represented Richard “R.J.” Vanecko after he was charged with manslaughter in the 2004 death of David Koschman — said his father played a large role in his decision to pursue a law career.

“They say imitation is the sincerest form of flattery,” he said.

Funeral arrangements for William Martin are pending.