An unsolved series is usually explained by the killer: he was clever, he was lucky, he stopped in time. Read a run of them together and a different variable comes forward, and it is not about him at all.
The Atlanta Ripper killed at least fifteen women in Atlanta in 1911 and 1912. They were Black women working as cooks, laundresses and domestic servants. Their throats were cut, several were mutilated, and the bodies were left arranged in the open where they would be found. The police of 1911 made little effort. The press treated the series as a curiosity. Several men were tried and acquitted or released, and no one was ever convicted.
The Freeway Phantom took six Black girls and young women off the streets of south-east Washington DC in 1971 and 1972 and left them beside highways. He put a note in one victim's coat. The case is still open. It was reopened in 2018 and again in 2023, which tells you both that it was never closed and that it was never worked to a finish.
The Doodler stabbed at least five men in San Francisco in 1974 and 1975, having sketched them in bars beforehand. Three men survived him. All three declined to testify rather than be identified publicly as gay, and the case went unprosecuted for decades. The obstacle in that case was not evidence. It was what testifying would cost the witnesses.
The Frankford Slasher killed eight or nine older women in Philadelphia between 1985 and 1990, several of them drinkers from the same few bars along Frankford Avenue, stabbed repeatedly in their own homes or the streets nearby and left partially undressed. One man was convicted of the final killing and maintains his innocence. The rest of the series is unsolved.
Set those beside the unsolved cases that everybody can name. Jack the Ripper killed five women in Whitechapel in 1888 and the name itself comes from a letter now generally thought to be a journalist's forgery. The Zodiac killed five in northern California in 1968 and 1969, claimed thirty-seven, and sent more than twenty letters and four ciphers, one of which held out until 2020. The Cleveland Torso Murderer left at least twelve dismembered bodies, several drained of blood, most of them in Kingsbury Run, and Eliot Ness ran the investigation. The Phantom Killer of Texarkana killed five in ten weeks in 1946 and the first survivors described a white hood with holes cut for the eyes.
The famous four are not famous for being harder. Zodiac and the Ripper are famous because the killer produced text, and text is what a newspaper can print. Cleveland is famous because Eliot Ness lost. Texarkana is famous for the hood.
The other four are not less documented. They are less remembered, and the pattern in who is remembered is not subtle: Black domestic workers in 1911, Black girls in 1971, gay men in 1974, older women who drank in the same bars in 1985. In each of those the investigation was thin at the time, and the thinness was not a mystery to anyone then either.
All eight remain open. What separates them is not how much is known about the killer. It is how much was done while it was still happening.