"This was a violent and brutal struggle," said Capt. James Clark, commander of the Homicide Unit.
Still unsure of a motive in the killing, homicide detectives returned to Ambruster's apartment in the 5500 block of Wayne Avenue on Thursday, trying to determine whether any of her collectibles or other belongings might have been stolen, he said.
There were no signs of forced entry in the second-floor apartment, Clark said, suggesting that Ambruster either knew her killer or was ambushed as she entered her building. A window was open, but police do not believe it was used to gain entry to the apartment.
Detectives were searching for evidence that Ambruster might have been expecting a guest, Clark said.
"We are looking through all that, but as of right now, we just have not found anything," Clark said. "Right now it's very much a whodunit."
He said investigators were reviewing footage from surveillance cameras in the apartment complex and the neighborhood that might have captured images of the assailant.
In a statement to police, Ambruster's roommate said that when he returned home from running errands, he found the door unlocked.
Clark, who visited the crime scene Thursday, said detectives were trying to locate close friends or relatives who might be able to tell them if something were missing from the apartment.
But that was proving to be a challenge as well, he said, since it appeared that Ambruster lived a mostly solitary life, with her only close relative a sister in California.
In an interview Thursday, Ariel Ambruster said her sister had a big heart, and called her death a shock.
"It's just so utterly horrific that I can't even digest it," she said.
Friends and colleagues described Ambruster as bright, warm, kind, and devoted to her three cats. The cats have been hiding in the apartment since the crime, said Ambruster's longtime friend David Carroll. "Those are her babies," he said.
Ambruster was fascinated by astronomy and continued to work on academic papers even in retirement. She made trips to New Mexico to study early astronomers' use of rock formations and buildings at Chaco Canyon.
Carroll met Ambruster when he was a student volunteer at the Franklin Institute in the late 1970s. Ambruster was working there while she earned her doctorate at the University of Pennsylvania.
As she became more interested in the history of astronomy, she was torn between moving to the Southwest and staying in Philadelphia, but her love of the city and its history held her here, friends and family said.
She enjoyed going to Phillies games and watching the Eagles. She liked to go bird-watching around Philadelphia and Cape May.
A convert to Judaism, Ambruster attended P'nai Or, a Jewish Renewal congregation in Mount Airy that incorporates Hasidic prayer and mystical teachings into its liturgy. Her roommate is a member of the congregation as well.
Rabbi Marcia Prager said Ambruster was active in the small congregation and would be missed.
"She was very loved and admired," said Prager, who described Ambruster as a private person who had made several close friends in the congregation.
On the day of her death, Ambruster spent part of the afternoon shuttling fellow congregants to a food co-op, said friend Tony Hull.
Hull said he spoke to Ambruster on Monday afternoon about an astronomy paper they were working on with a colleague. She was excited about going to Washington to present the paper next month at the American Astronomical Society's annual meeting, he said, but worried that the paper was not as complete as it should be.
"Carol was very much a perfectionist," said Hull, an adjunct professor at the University of New Mexico.
Ambruster told Hull she was busy running errands and would call him back.
"She said, 'I'll talk to you later,' " Hull said, "and of course, it didn't happen."
Contributing to this article were Inquirer staff writers Julie Zauzmer, Kristen A. Graham, and Robert Moran.