WATCH: Why Experts Think the Long Island Serial Killer — or Killers — Is a ‘Seasonal Hunter’

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It’s been nearly six years since police discovered that a serial killer may be afoot on Long Island, New York.

On Monday night, PEOPLE’s crime team takes a closer look at the enduring mystery with the premiere of its new true crime series on Investigation Discovery, People Magazine Investigates.

Despite years of investigation, there are more theories than answers about who is responsible for at least some of the 10-plus bodies discovered around Long Island’s Gilgo Beach. The case has since been dubbed the Long Island Serial Killer.

Is only one person responsible for all of the dead? What connects them all? Because many of the victims vanished in the summer months, could the killer or killers be a teacher — or perhaps a fisherman?

“He is a seasonal hunter, ” says serial killer expert and criminal profiler John Kelly in an exclusive clip from People Magazine Investigates‘ Monday night premiere, which will focus on the case.

“Where does he go for the rest of the year? That is the question.”

• Watch our new 10-part true crime show, People Magazine Investigates,which debuts with the two-hour season premiere “The Long Island Serial Killer” on Monday, Nov. 7, at 9 p.m. ET on Investigation Discovery

There are clues as well — such as the ways in which some of the victims were killed, and how they were stalked before dying — and it’s these pieces of information that have shed some light for investigators.

In the new clip from Monday’s premiere, Mary Ellen O’Toole, the director of the forensic science program at George Mason University and a retired FBI agent and profiler, says the killer is a “puppeteer” who “knows in the end what he is going to do and she has no idea.”

• Watch the People Magazine Investigates After Show, available Nov. 7 on the new People/Entertainment Weekly Network (PEN). Go to PEOPLE.com/PEN, or download the PEN app on Apple TV, Roku Players, Amazon Fire TV, Xumo, Chromecast, iOS and Android device

“He is involved in predatory behavior, and he is enjoying the hunt,” O’Toole says.

“We are talking about a whole different breed of person here,” she says. “He has the verbal skills to be able to communicate in a way that is not threatening.”

People Magazine Investigates premieres Monday (9 p.m. ET) on Investigation Discovery.