What really drove the ‘Grim Sleeper’ nickname, and what was hidden during that CHILLING DEAD SILENCE?

He terrorized the streets of South Los Angeles over a 22-year period, killing at least 10 women from 1985 until 2007.

He was nicknamed the “Grim Sleeper” serial killer because of an apparent gap in the killings from 1988 until 2002, but for years his identity remained unknown as police made little progress in their investigation.

Finally, in 2010, familial DNA testing of an arrested man revealed the killer to be Lonnie Franklin, a married father-of-two and a former LAPD mechanic and sanitation worker for the city of Los Angeles. (The arrested man whose DNA was tested was Franklin’s son, Christopher.)

Earlier this year, Franklin, 64, was tried, convicted, and sentenced to death for the murders. The case is the focus of the upcoming People Magazine Investigates episode “The Grim Sleeper,” which airs Monday night at 10 p.m. ET on Investigation Discovery.

AP Pool/Nick Ut,Pool

Ahead of that broadcast, PEOPLE Senior Editor Alicia Dennis sat down on Friday’s People Now to preview the episode and talk about aspects of the case.

For instance, Dennis explains that PEOPLE crime reporter Christine Pelisek nicknamed Franklin because of the apparent gap in the killings. But she adds, “What’s chilling is that perhaps there wasn’t a gap,” and that Franklin may have killed more women whose bodies have not been discovered.

Dennis also explains that many of Franklin’s victims were marginalized people, which perhaps contributed to how long the case took to solve.

“His victims were women that he picked up on the street,” Dennis says. “And some of them were prostitutes, others were not.”

In the People Magazine Investigates episode, Pelisek, whose investigative reporting on the case has been roundly acclaimed, explains that Franklin’s benign appearance made him a more dangerous killer.

“None of these girls would have gotten into the car [with Franklin] if they thought he was threatening,” she says. “He obviously comes across as not being threatening. But he’s deadly.”

People Magazine Investigates: The Grim Sleeper airs Monday night at 10 p.m. ET on Investigation Discovery.