Three checks came back clean. All three men killed again.

What did Gary Ridgway’s polygraph show?

Gary Ridgway passed a polygraph examination in 1984 in which he denied the killings. King County prosecutors later wrote that he had already killed scores of women by then and that agreeing to the test helped him avoid suspicion. Police arrested him on November 30, 2001.^4,5,13

Marie Malvar, 18, disappeared on April 30, 1983.^4,13 Her boyfriend had seen her get into a pickup truck, and he later told police he thought he had found it. It was Gary Ridgway’s.^1,4 Ridgway, known in the press as the Green River Killer, denied picking her up.^1,4,5

In 1984 Ridgway passed a polygraph examination in which he denied killing any of the women.^4,5 King County prosecutors later wrote that he had already killed scores of women by the time of the test.⁴ Agreeing to be interviewed without a lawyer and to take the polygraph, they wrote, helped him avoid suspicion.⁴

The task force searched his home on April 8, 1987, and took a saliva sample.^1,4 In March 2001 the evidence went to the state crime lab for DNA testing.^1,4 Police arrested him on November 30, 2001.^4,13 He pleaded guilty to 48 murders in 2003 and to a 49th in 2011.^1,5

The plea covers women killed after the polygraph.^4,13 Marta Reeves disappeared in 1990.^4,13 Patricia Yellowrobe was found dead on August 6, 1998.^4,13 Prosecutors wrote that he admitted he kept killing until at least 1998.⁴

Why did police release Andrei Chikatilo after holding him in 1984?

Police detained Andrei Chikatilo in Rostov-on-Don in September 1984. Investigators believed the killer’s blood group was IV, and a test recorded his as group II. He was released and went on killing until his arrest in November 1990. Investigators later found that the group II result was a technical error.^2,6,7,11

Svetlana Korostik was killed on November 6, 1990.^7,11 Police arrested Andrei Chikatilo on November 20, 1990.^6,7 They had held him in 1984 and let him go.^2,7

In September 1984 police detained him in Rostov-on-Don one morning, after he had spent the night approaching women.^2,6 A knife and rope were found in his briefcase.^6,11

Investigators believed the killer’s blood group was IV.^7,11 A test recorded Chikatilo’s as group II, and the result appeared to rule him out.^2,7,11 He was convicted of stealing state property and sentenced to a year of correctional labor.^6,11 No further check was made on him.^6,11

After his 1990 arrest, investigators found that the group II result had been a technical error.^2,6 Lenta.ru, citing the trial records, reports that an expert commission confirmed this in August 1991.⁶ A Rostov court convicted him in October 1992 of 52 murders of women and children.^8,12 Russia’s Supreme Court later set aside several of the murder counts as unproven and upheld the death sentence, and more than forty murder convictions stood.^6,7,11

What did police check before clearing Harold Shipman?

Winifred Mellor, Joan Melia and Kathleen Grundy were patients of Harold Shipman, a family doctor in Hyde, near Manchester.^3,9 The Shipman Inquiry found that he killed all three after Greater Manchester Police closed an investigation into him on April 17, 1998.³ The Inquiry dates their deaths to May 11, June 12 and June 24, 1998, in that order.³

The Inquiry found that on March 24, 1998, Linda Reynolds, a doctor at a neighboring practice, took her concerns about the number of Shipman’s patients who were dying to the coroner.³ She reported that he had signed 16 cremation forms in three months, while her practice had 14 deaths in the same period.³ The Inquiry noted that her practice had about three times as many patients.³ A funeral director had raised concerns with her as well.³

The Inquiry found that the detective inspector running the investigation made no check on the Police National Computer, which would have shown Shipman’s convictions for drug offenses committed in the early 1970s.³ A medical adviser to the health authority examined the patients’ medical records.^3,10 The Inquiry found that his prior knowledge of, and respect for, Shipman made him unsuitable for the task, and that his opinion effectively ended the investigation.³

Shipman was arrested on September 7, 1998, the Inquiry records, after Kathleen Grundy’s daughter told police she suspected a forged will.³ He was convicted in January 2000 of murdering 15 patients and forging a will.^3,9 The Inquiry judged it probable that the lives of his last three victims would have been saved if the police and the coroner had moved with reasonable expedition.^3,10

What do the three checks have in common?

Each check answered a narrower question than the one that mattered. Prosecutors wrote that a polygraph is a test designed to detect stress, and that Ridgway’s psychopathy carried him through it.⁴ A blood test is only as good as the group the lab records, and in 1984 the test recorded the wrong one for Chikatilo.^7,11 The Shipman review rested on a reviewer the Inquiry found unsuitable for the task.³

Each clean result then became a fact in the file. In two of the three cases the evidence was read correctly only years later: a saliva sample taken in 1987 and tested in 2001, and a blood group checked again after the 1990 arrest.^1,2,4