When Hull heard of the arrest, he said, his reaction was "just sadness that someone could be so screwed-up to do something like this.
"This whole thing still just feels like a nightmare."
Police said Saturday they had no new information to release. They have not released a motive for the Dec. 9 killing or information about any connection between Ambruster and Diaz.
Ambruster's roommate discovered her body with a knife in her neck in the kitchen of her second-story apartment in the 5500 block of Wayne Avenue.
She had been stabbed multiple times in the neck and chest.
There were no signs of forced entry, detectives said. Police sources said homicide detectives had interviewed a subcontractor who had access to the building. They were zeroing in on him and waiting for lab results of blood found on his clothing, the sources said.
On the 100 block of West Hansberry Street, where Diaz lives, a few blocks from Ambruster, friends and neighbors on Saturday said they were surprised by the arrest.
"He wasn't a known threat. It was a shock to the neighborhood," one man said. Others in the group agreed, calling Diaz "the handyman of the neighborhood."
Diaz worked in construction, neighbors said, and they said they believed he had been working for Ambruster. He got along with others in the neighborhood, they said, and would do handiwork as needed - shoveling snow, pest control, replacing lightbulbs.
He had lived in the Germantown home for a few years.
"I never would have thought that he would have did something like that," said a woman at the corner store where Diaz had helped out. "He was always friendly. He used to come in here and joke."
Shortly after noon, a police officer who had helped serve a search warrant came back to look around Diaz's front porch. Taking off his gloves, the officer declined to comment, saying he was there only for a quick follow-up, before driving off.
A 68-year-old woman living next door who asked not to be named said she believed Diaz had been trying to avoid trouble with the law.
"One time, he told me he didn't want to go back to jail," she said.
She said she had asked Diaz what was going on when police were investigating before the arrest.
"I knew that they were searching his house," she said. "I asked him what happened. He said, 'Nothing,' he did nothing wrong."
Inquirer staff writer Michaelle Bond contributed to this article.