Vincent Bugliosi, who prosecuted Charles Manson and his followers, shown in 2013 with his book “Reclaiming History: The Assassination of President John F. Kennedy.” (Damian Dovarganes/AP)

Vincent Bugliosi, the Los Angeles prosecutor who won convictions against Charles Manson and several of his followers for a series of heinous murders in 1969 and who later wrote a best-selling true-crime book, “Helter Skelter,” about the Manson cult and the killings surrounding it, died June 6 in a Los Angeles hospital. He was 80.

The cause was cancer, his wife, Gail Bugliosi, told the Los Angeles Times.

Mr. Bugliosi (pronounced bool-YOH-see) was a deputy district attorney when he was asked to prosecute some of the most gruesome and unsettling killings in the country’s history.

“When you talk about the Manson case,” he told the Los Angeles Times in 1994, “you’re talking about perhaps the most bizarre murder case in the annals of crime.”

In the early-morning hours of Aug. 9, 1969, several people entered a Los Angeles estate rented by the film director Roman Polanski, who was in Europe at the time. Polanski’s wife, 26-year-old actress Sharon Tate, was at the house with several friends.

The next day, the body of Tate, who was eight months pregnant, was found stabbed and hanged. The four houseguests were also killed, along with a teenaged boy who was visiting the estate’s caretaker. The word “Pig” was scrawled on a door in the victims’ blood.

The following night, grocery-chain owners Leno and Rosemary LaBianca were tied up and slain at their home. The phrases “Death to Pigs” and “Healter Skelter” — misspelled by the killers — were written in blood.

The two incidents were not linked by police until one of Manson’s followers, Susan Atkins, in jail on another charge, told fellow inmates about the killings. Manson was arrested at a ranch outside Los Angeles, where he presided over a band of drifters known loosely as the Manson Family.

The Tate-LaBianca murders, as they became known, cast a pall of fear across Los Angeles and the country.

“There were areas of the city where folks literally did not lock their doors at night,” Mr. Bugliosi told Newsweek in 2009. “That ended with the Tate-LaBianca murders. The killings were so terribly brutal and savage: 169 stab wounds, seven gunshot wounds. They appeared to be random, with no discernible conventional motive.”

Working 100-hour weeks, Mr. Bugliosi made the case against Manson and his co-defendants, Atkins, Patricia Krenwinkel and Leslie Van Houten. Although Manson was not present for the murders, Mr. Bugliosi argued that he was the ringleader and compelled others to commit murder on his orders.

Many strange things occurred during the nine-month trial. Manson and his female co-defendants showed up in court with “X’s” carved into their foreheads and later appeared with shaved heads.

At one point, the 5-foot-2 Manson leaped over a table and lunged toward the judge with a sharpened pencil, shouting, “In the name of Christian justice, someone should chop off your head.”

One of the attorneys for the defense team disappeared during the trial and was later found dead, believed to be yet another Manson murder victim.

In January 1971, Manson and his co-defendants were found guilty of murder and conspiracy to commit murder. Another associate, Charles “Tex” Watson, was convicted in a separate trial. All were sentenced to death, but after the California Supreme Court outlawed the death penalty in 1972, the sentences were converted to life in prison.

In 1974, Mr. Bugliosi and a co-author, Curt Gentry, published “Helter Skelter,” a detailed account of the trial and the Manson cult. The book sold millions of copies, inspired two TV movies and is often called the best-selling true-crime book of all time.

Mr. Bugliosi said Manson’s goal was to provoke racial warfare — which he called “Helter Skelter” — and somehow emerge from the chaos as a supreme leader.

“The very name Manson has became a metaphor for evil,” Mr. Bugliosi told the Los Angeles Times in 1994. “He has come to represent the dark and malignant side of humanity, and for whatever reason, there is a side of human nature that is fascinated with ultimate evil.”

Atkins died in 2009. Manson and the other convicted killers are still behind bars.

Vincent Bugliosi Jr. was born Aug. 18, 1934, in Hibbing, Minn. His father ran a grocery store and was a railroad conductor.

Mr. Bugliosi won the Minnesota state high school tennis championship in 1951, then spent his senior year of high school in Hollywood, Calif. He won a tennis scholarship to the University of Miami, from which he graduated in 1956. He served as an Army officer, then graduated from law school at the University of California at Los Angeles in 1964.

As a prosecutor, Mr. Bugliosi reportedly won 105 of 106 trials and never lost a murder case. He made several failed attempts to be elected Los Angeles district attorney and California attorney general in the 1970s.

Survivors include his wife of 58 years, Gail Talluto Bugliosi of Pasadena, Calif.; and two children.

Mr. Bugliosi practiced law until the mid-1980s while turning out more than a dozen books, most of them bestsellers. He wrote about why he believed O.J. Simpson “got away with murder,” why the existence of God could not be proved and, in “The Betrayal of America” (2001), why he believed the Supreme Court’s decision affirming George W. Bush’s election as president in 2000 was unlawful.

Besides “Helter Skelter,” Mr. Bugliosi’s best-known book was “Reclaiming History” (2007), a 1,600-page analysis of the assassination of President John F. Kennedy and its subsequent investigation. He spent 21 years on the book before reaching his firm conclusion: Lee Harvey Oswald was the lone assassin, and there was no conspiracy.

Nonetheless, Mr. Bugliosi knew that he would forever be identified with the Manson case, and the eternal fascination with evil. He recalled that he once shared a cab ride with Pulitzer Prize-winning historians William Manchester and Arthur Schlesinger Jr.

“That’s all they wanted to talk about,” Mr. Bugliosi told the Los Angeles Times in 2009. “Tell me about his eyes. Did you ever talk to him? How did he get control over these people?”