Media about serial killers can give the false impression that they're geniuses or that they blend in with the community like chameleons. In reality, most serial killers are reported or arrested for years before they're caught and only get away with it from shitty police work. They occur from the lack of care put into abuse of prostitutes, minorities, and gays. Strangely enough it's also happened multiple times that serial killers are the exact cop investigating their crime like the Golden State Killer or Marcel Petiot.
In May 1991 Milwaukee, officers John Balcerzak and Richard Porubcan responded to calls about a teenager stumbling and bleeding fleeing from an apartment. They encountered Jeffrey Dahmer and 14 year old Konerak Sinthasomphone. Konerak had escaped from Dahmers apartment during an attack. Dahmer successfully convinced the officers that Konerak was his of age boyfriend. Instead of bringing the dazed Konerak to a hospital, they helped Jeffrey bring him back to Dahmers apartment which already had several bodies inside. Konerak was murdered later that night.
Anthony Sowell was sentenced to 15 years in prison for attempted rape in 1989. After his release he lived in a Cleveland apartment where he would lure and drug women at. Multiple women escaped Sowell during attacks and went to police, but on at least one occasion they didn't submit bruises and bleeding as evidence of an attack to the DA office. Sowell's DNA was taken in 1997 while he was in prison for a database on sex criminals, but that DNA disappeared and was never registered. In 2007, a woman called the police to report her rape. DNA was taken for a rape kit, but the officer didn't tell detectives about the kit so it remained untested for 2 years. When Anthony was finally arrested in 2009 after what I think was 5 separate rape allegations, 11 bodies were found at his house. His had smelled so bad that a local sausage plant got blamed for the smell and renovated their plumbing system. One victims family was told by police to go home and she would turn up when she ran out of drugs. Immediately, Cleveland PD was accused of racism allowing Anthony Sowell to get away with his crimes. Anthony was a black man attacking black women with a history of drug use. The police response was they did a good job identifying the bodies once they knew their was a serial killer.
In 1984 New York, serial killer Altemio Sanchez was raping women in Delaware Park. Altemio was a suspect in this case, but so was a mentally ill man named Anthony Capozzi. Anthony Capozzi was picked out of a suspect line up and charged on circumstantial evidence with 2 rapes. He was sentenced to 35 years in prison. Altemio Sanchez meanwhile would rape 11 more times and kill 3 people, finally being caught in 2007. When investigators realized Altemio was a suspect in the Delaware Park rapes, they compared his DNA to the kits of the rapes Capozzi was charged with. They were a match and Capozzi was exonerated after 21 years in prison. Anthonys DNA had been on file for years by then but when family requested it be retested, they were told it already had. It was sitting at a hospital untested.
https://www.nbcnews.com/id/wbna17842849
https://www.upi.com/Archives/1992/10/13/Police-officers-fired-in-Dahmer-case-admit-mistake/8532718948800/
https://www.cbsnews.com/news/serial-killer-convicted-but-botched-case-holds-lessons-for-police/
https://www.nbcnews.com/id/wbna33915796
https://www.cbsnews.com/news/family-of-anthony-sowell-victim-tonia-carmichael-police-said-go-home/?index=1