That person, described by ABC News as a family friend of 36-year-old Jason Smith from "many years ago," asked for anonymity.
That friend said Smith, of Levittown, had behavior and anger issues, and that he also liked to set things on fire.
Police sources told Action News that Smith confessed to killing 35-year-old Melissa Ketunuti. He was charged on Thursday morning remains behind bars and is scheduled to have a court hearing next month.
Smith appeared before a magistrate at the Criminal Justice Center wearing a grey Yankees hoodie on Thursday night.
The judge asked him if he understood the charges against him, to which he replied, "Yes, sir."
Shortly thereafter, Smith was taken to prison to await his preliminary hearing.
Ketunuti was taking part in a pediatric fellowship at Children's Hospital of Philadelphia. Her friends and coworkers paid tribute to her in an emotional vigil, also held Thursday night.
It was a solemn vigil in front of a sea of candles, which marked virtually every inch of the stoop in front of Dr. Ketunuti's row home.
The candles were placed there by her loving colleagues at Children's Hospital of Philadelphia, a place they feel fortunate to work at.
"The opportunity to work with amazing people, people like Melissa Ketunuti," colleague Dr. Levon Utidjian said.
Earlier, they had braved bitterly cold temperatures to walk almost two miles from Children's Hospital to the doctor's home on the 1700 block of Naudain Street.
Even nearby residents were moved by the touching act.
"I just walked by for a moment and saw the lights. It makes you start crying without even knowing her," Center City resident Julia Bringhurst said.
Ketunuti was found strangled, bound and burned inside the basement of her home in Center City on Monday. Smith is charged with murder, arson, abuse of a corpse and causing/risking a catastrophe.
Police say Ketunuti had called an exterminator service because of rodents in her home, and that Smith, a subcontractor, was dispatched.
Action News has learned Smith wasn't licensed to perform that job as required by the state.
Once in the home, police said, there was some kind of argument in the basement. According to sources, Smith told police Ketunuti had "belittled" him.
The argument escalated, police said, and Smith allegedly knocked Ketunuti down, got on top of her, and strangled her with a rope that was in the basement. Police also said Ketunuti's arms and legs were bound at some point during the attack.
Police say after Smith killed Ketunuti, he lit some papers on fire at the kitchen stove and then went back downstairs and set her body on fire in an attempt to hide any evidence of his DNA.
But, police say, while Smith may have thought he was destroying DNA evidence by setting the fire, he may have left his hooded sweatshirt behind. Police are now analyzing one that was found in the basement.
After the crime, police say he went on to another job in New Jersey.
At a news conference on Thursday morning, Captain James Clark of the Homicide Unit said investigators locked on to Smith as a suspect because he was seen on surveillance video in the area of Ketunuti's home.
"After the murder, we see him get into the truck and go down Naudain Street twice, so that led us - even more - to the suspect," Capt. Clark said.
Smith was taken into custody around 9:00 p.m. Wednesday at his Levittown home.
At some point, a dog that was at the house was shot by police. Action News was there when the dog's body was carried off Thursday morning.
Captain Clark said Smith had nothing more than traffic offenses on his record and a DUI in 2004. He also said that Smith and Ketunuti had never met before, and Monday was the first time Smith had been at the home.
"You have a young physician who dedicated her life to helping people and, to have her die in this manner, is very sad," Captain Clark said.
Action News spoke to the mother of Smith's fiancée on Thursday. She says that the whole family is distraught. She is devastated that the police came into the home last night, shot their dog, and then took Smith away. She says the family did not know that he was wanted for the murder. She said they had no idea that he could have done something like this.
Police said Thursday that Ketunuti's family was on the way to this area from Thailand.