On Tuesday night, Paul Haynes joined Patton Oswalt and Billy Jenkins for an event in Naperville, Illinois, celebrating I’ll Be Gone in the Dark, the best-selling book by Oswalt’s late wife, Michelle McNamara, that the three men had assembled from her unfinished manuscript after her death. The book, about McNamara’s search for the Golden State Killer, a serial rapist and murderer who terrorized California in the late 1970s, has an indefinite ending. The perpetrator had not been caught, and McNamara could only imagine the day he might be, in a letter she wrote to the man whose name she would never learn. McNamara was born and raised in Chicagoland, lending the evening a bittersweet air; it would have been even more emotional if those gathered had known that mere hours later the Sacramento sheriff’s department would arrest the man they believe is responsible for the crimes, Joseph James DeAngelo.

Sacramento County District Attorney Anne Marie Schubert, joined by law enforcement officials and prosecutors from five California counties, led a press conference on the arrest Wednesday. Not much information was imparted. Schubert said that the investigation had come together over the past six days and that DeAngelo had been identified as the Golden State Killer (also known as the East Area Rapist/Original Night Stalker, or EAR/ONS) after investigators surveilling his home in the Citrus Heights district obtained discarded DNA evidence for analysis. DeAngelo has lived in the house, which neighbors told local news reporters he shared with a woman and two teenagers, for 30 years. (DeAngelo married Sharon M. Huddle, an attorney, in 1973.)

“I’m exhilarated,” Haynes said in a phone call Wednesday afternoon. He has made a study of the Golden State Killer case for the past seven years, four of them working with McNamara as her lead researcher. He’s not chagrined to learn that DeAngelo was not on any of the myriad lists of suspects compiled by both law enforcement and civilian sleuths. “I knew that would be the case,” he said, “somebody on nobody’s radar who I never would have found.”

The arrest has produced some surprises nonetheless. DeAngelo, Haynes’ contacts have suggested, has admitted to being the Visalia Ransacker, an oddly behaved serial burglar and Peeping Tom who, in 1975, shot and killed journalism professor Claude Snelling when Snelling halted his attempt to abduct Snelling’s teenage daughter. “I had reached the point where the two series didn’t seem connected to me,” Haynes said, and there was no DNA evidence left at the Visalia crimes to suggest otherwise.

The Golden State case has perplexed law enforcement for decades. Although the crimes were committed before forensic science employed DNA analysis, investigators in the 2000s used it to determine that the same man was responsible for both the East Area Rapist assaults and a series of home invasion rapes and murders in Southern California. But they could not find a match in any DNA database. Some hoped that the geographic distribution of the crimes could be used to unearth potential suspects. But, Haynes said, “the geographic profiles done on this offender were completely wrong.” That’s because, he speculates, DeAngelo had been a police officer for the Northern California cities of Exeter and Auburn from 1975, when his crime spree began, through 1979, when he was fired after being caught shoplifting a can of dog repellant and a hammer from a Sacramento drug store.* (How the Golden State Killer managed to fend off his victims’ pets has long been a subject of speculation.) The crimes were committed in other counties, unusually far away for this kind of criminal. “The only circumstances where that would make sense in terms of geographic profiling,” Haynes said, “is if he were a cop and didn’t want to offend in the jurisdiction where he worked.”

Some psychological profilers inferred that the Golden State Killer’s habit of stalking and gathering information on his targets meant that he lived alone, but Haynes points out, “Who else but a cop could be out prowling every hour of the night, and not have that questioned by his wife?” Other small details that have suddenly acquired new significance include the report of a neighbor who said that on the night of one attack, she heard the sound of a police radio from her window. Survivors of some of the rapes reported that their attacker became highly emotional, weeping and speaking to “Mommy,” but one witness insisted that the name he used was “Bonnie.” Amateur sleuths on Reddit quickly dug up a newspaper notice from 1970, announcing the engagement of DeAngelo to a woman named Bonnie Jean Colwell. There is no record of the two ever marrying.

Perhaps the biggest mystery now is just how law enforcement identified DeAngelo as a person of interest in the crimes 40 years after they began. So far, officials have declined to explain, but Haynes has his suspicions. During the press conference, Sheriff Scott Jones said, “Although it was DNA ultimately that led us down the road, there was a lot of places that road could have led.” Haynes thinks it likely that investigators used DNA markers posted on genealogy websites to identify a possible ancestor of the killer and then followed the ancestor’s family tree down to the present, looking for male descendants who fit the profile. But this, he cautions, is only an educated guess, and what evidence law enforcement used to pinpoint DeAngelo is “the No. 1 thing I want to know.” [Update, April 26, 10:45 p.m.: The Sacramento Bee reports that Haynes’ suspicion was correct, according to the Sacramento Country District Attorney’s Office. DeAngelo was identified through genealogical websites.]

There are, of course, also “300 questions I want to ask this person,” Haynes said of DeAngelo, who is reportedly cooperative but on suicide watch. Whether he will ever get to ask those questions no one can say. “I’m just so sad,” he added, “that Michelle wasn’t here to see this.”