Fridays announcement that police had made an arrest in the infamous Long Island serial killings was shocking not just because of the incredible development in a cold case but also because the suspect like those at the center of so many recent high-profile cases lived an unassuming life near his victims, had no known history of violent crime, was seemingly successful and presumably didnt raise alarm bells among his family and friends.

Despite numerous podcasts, books and films about the killings and hordes of citizen sleuths speculating on message boards and social media the case had remained unsolved since the December 2010 discovery of the bodies of Melissa Barthelemy, 24, Amber Lynn Costello, 27, Megan Waterman, 22, and Maureen Brainard-Barnes, 25, who died in the summers of 2009 and 2010.

So what led investigators to surround and arrest 59-year-old architect Rex Heuermann on Thursday as he strolled down Fifth Avenue after leaving his Manhattan office at 8:30 p.m.? Authorities on Friday revealed how a task force combed through evidence over the last year, and on Monday they continued to seek any potential connections to the killings in personal belongings which included, per police, more than 200 firearms at his Massapequa Park home and storage units in the area. So far, Heuermann has pleaded not guilty to six charges in connection with the deaths of three women.

As police continue to unspool crime scene tape near Heuermanns home and other areas of interest, heres what we know about the Gilgo Four investigation and the many questions we have about the unsolved deaths of at least seven other people, many of them unidentified, whose remains have been found in desolate areas of Long Island.