At his high school in Sherrill, N.Y., Markoff joined the bowling team, played golf and was even active in the school's "youth court." His former English teacher said she was stunned when she saw Markoff had been picked up for the arrest.
"I saw his picture and that beautiful smile. I knew it was Phil," Sonja Hluska told "Good Morning America." " I just couldn't believe it was Phil. He was a bright student, so friendly, polite, teacher's pet."
"I thought it would be some derelict," she said. "Not this young man, because he's just such a good person."
ABC News has learned that Markoff and McAllister had reservations to stay at the Foxwoods Casino this weekend, the same casino where he reportedly won $5,300 just two days after Brisman was murdered.
Markoff was arrested Monday during a traffic stop, en route to Foxwoods with McAllister in the car, law enforcement officials told ABC News. Sources told ABC News he had scratches on his body at the time of his arrest.
Markoff was charged by Boston police for Brisman's murder and the robbery of another woman, Trisha Leffler, also in a Boston hotel, two days earlier. He's also being eyed by Rhode Island police for the Warwick Holiday Inn Express robbery.
He has pleaded not guilty.
Police have asked anyone who may have been contacted by Markoff via Craigslist to come forward, and law enforcement sources told ABC News that detectives investigating the case are now fielding calls from other potential victims.
Connecticut Attorney General Richard Blumenthal is calling for the erotic services portion of Craigslist shut down, something other state officials across the country have also started looking into.
"We have a responsibility to protect children and women and anyone who may be victims of these kinds of criminal activity that may involve human trafficking, child exploitation other brutal and violent crime," Blumenthal said.
In an interview with the Boston Globe, Craigslist CEO Jim Buckmaster denied the site offers sex-related services, despite the "erotic services" portion. Police said that's where Markoff found all of his alleged victims.
Police said they tracked Markoff down using surveillance videos at three hotels. In photos taken from hotel cameras, the man police have been seeking in connection with the crimes appears to be fixated on a BlackBerry. Investigators said that Blackberry is what led them to Markoff.
A police source close to the investigation said cops found Markoff by tracing Craigslist e-mails.
"They followed high-tech leads and old-fashioned shoe leather," Suffolk County District Attorney Dan Conley said. "They've connected IP addresses and physical locations."
ABC News' Scott Michels contributed reporting to this story.