He circled Naudain twice, then was gone.
Fifteen minutes later, a dog walker pushed open Ketunuti's unlocked front door to the smell of smoke and the gruesome discovery of Ketunuti, a 35-year-old pediatrician and researcher at Children's Hospital of Philadelphia, bound and strangled in her basement, her body set afire.
Surveillance footage captured Monday by cameras atop a southwest Center City hospital and coffee shop provided police with an outline of a shocking killing - and eventually to the arrest of Jason Smith, the 36-year-old Levittown exterminator charged Thursday with Ketunuti's killing and abuse of her corpse.
After viewing the security footage, including some from cameras outside Penn Medicine Rittenhouse at 18th and Naudain Streets, members of a homicide task force immediately wanted to find the workman who had seemed to follow Ketunuti down Naudain.
They were "relentless" in tracking him down, Capt. James Clark said Thursday at a news conference.
Through photo-imaging technology, by early Wednesday task force members had matched the face in the video to Smith's Bucks County driver's license, police sources said.
Ketunuti's boyfriend, Surya Mundluru, a surgeon who lives in New York, told investigators that Ketunuti had planned to schedule some home-improvement appointments, including plumbing work in the basement and a visit from an exterminator.
Tracing Ketunuti's cellphone calls and reviewing her text messages, investigators found she had made an appointment with an extermination company. The number came back to Smith's phone.
Smith was watching American Idol with his girlfriend and their young daughter Wednesday night when police descended on the powder-blue remodeled two-story house the three share with his girlfriend's mother and stepfather. Red, white, and blue bunting hung in the driveway, and wire-mesh Christmas reindeer decorated the front lawn.
The family dog, a boxer named Tyson, charged at police, who shot and killed it.
Smith's F-150, parked outside, was searched and towed. From it, crime-scene investigators recovered Smith's work bag, jacket, clothing, and gloves, police sources said. Investigators also searched Smith's trash, picking through it next to a baby pool in the white-picket-fenced backyard as police helicopters hovered. They also removed Smith's home computer, which he used to schedule his exterminating work.
His girlfriend told investigators he often worked extermination jobs in Center City.
Smith told detectives he visited Ketunuti's home Monday after she booked an appointment with Dave Bilyk Exterminators, a pest-control company in Newtown for which he was a subcontractor.
He had heard about Ketunuti's killing, he told investigators, but said he did not contact police about his visit to her home, fearing they would falsely suspect him of the crime because of past breaking-and-entering convictions. (Smith's criminal history lists only a 2004 DUI conviction.)
Smith confessed after a few hours, at times seeming solemn, Clark said, but at other times as though in a fog.
Smith told police he is addicted to prescription medicines, including painkillers.
He said he "snapped" after a brief argument in the basement with Ketunuti over something to do with the work, police sources said. He claimed that Ketunuti - a petite woman whom neighbors described as ever-friendly and gracious - said something that "belittled" him, according to police.
Investigators cast doubt on Smith's version, finding it more likely he attacked Ketunuti after trying to steal something from the house or possibly after making a sexual advance.
"He struck her, knocked her down. He got on top of her and strangled her," Clark said.
Smith said he used a lighter to "light her clothes," according to police sources.
Investigators determined that Ketunuti's body was burned after she died, based on the absence of soot in her lungs.
Police had yet to find any traces of accelerant on Ketunuti's body, sources said, but were testing the exterminating chemicals recovered from Smith's truck to see whether they were used to set the fire.
Smith told investigators he lit the fire in a panic. "The only thing I can surmise," Clark said, "is that he was trying to hide evidence or whatever DNA we may or may not be able to get."
To bind and choke Ketunuti, Smith used rope he found in her unfinished, concrete-dust-covered basement.
It was not clear whether Smith tied up Ketunuti while she was still alive or after she died in a possible attempt to move her body before setting it afire.
Smith had never done work at Ketunuti's home. "It was the first time they had ever met," Clark said.
Ketunuti's parents, who live in Thailand, were still on their way to Philadelphia, Clark said.
Homicide task force members were still working Thursday to collect evidence from Ketunuti's rowhouse and Smith's home.
Smith was arraigned via video Thursday night. He was advised of the charges against him and introduced to his court-appointed lawyer, James A. Funt.
Smith was impassive through the five-minute proceeding, saying little beyond "Yes, sir," in response to questions. He was ordered held without bail, and a preliminary hearing was set for Feb. 13. Funt declined to comment after the arraignment.
Contact Mike Newall at 215-854-2759, firstname.lastname@example.org, or follow @MikeNewall on Twitter.
Inquirer staff writers Robert Moran and Allison Steele contributed to this article.