Last week, 92-year-old Thelma Steele let a teenager who grew up across the street into her Forestville home, police said. They said the 15-year-old stabbed and beat her, leaving her on a sofa to die.

William Roger Fitts was arrested Wednesday and charged as an adult with first-degree murder, Prince George’s County police said.

Acting Deputy Police Chief Craig Howard said Fitts’s primary motivation appeared to be robbery. The teen made off with some of Steele’s property, although investigators were trying to determine exactly what was taken, Howard said.

Howard said investigators searched Fitts’s home Wednesday and took the teenager into custody, along with four other people. He said Fitts eventually gave a statement “implicating himself as the sole suspect” in the crime. The teenager told detectives that a knife found in his house was the murder weapon, Howard said. The four other people were let go, Howard said.

Law enforcement sources said someone had tried to use a credit card belonging to Steele, which helped lead investigators to Fitts.

Steele was found dead Monday night in her home on East Avenue after relatives asked police to check on her, authorities said. Neighbors had contacted Steele’s family when they noticed mail piling up outside the house.

News of Steele’s death shocked relatives and neighbors, who struggled to understand why someone would kill a sweet woman who neighbors said rarely ventured outside her house.

Steele, who worked as a secretary at a bank before she retired, had lived alone for years on East Avenue, said Wendy Tucker, a granddaughter. She did not remarry after her husband died of a heart attack more than three decades ago, and she lived independently for the most part, using cabs or a bus service to go to the grocery store, Tucker said.

Tucker said her grandmother’s home had been ransacked, and Howard confirmed that information Wednesday.

“We can’t even figure it out,” Tucker said Tuesday before news of the arrest. “We’re devastated.”

Tucker said that relatives had not talked to Steele for several days and that they were concerned when they could not contact her Monday. She said that her grandmother’s house was burglarized earlier this year when someone with a flashlight crept into the bedroom and took some jewelry while Steele pretended to sleep. Police spokesmen confirmed that detectives had investigated a burglary at the house in February. They said Fitts had not been tied to that crime or any other break-ins in the area.

Tucker could not be reached Wednesday night, but neighbors said Fitts’s arrest was puzzling. They said the Fittses were a sweet and caring family that lived in two houses across from Steele’s. William Fitts’s sister Connie would often check up on her elderly neighbor, they said.

William Fitts grew up on East Avenue, and he recently started to grow facial hair, said Joanna Johnson, 39, a former neighbor. Johnson said he rode bikes with her daughter, Joy, when the two were little.

“I wouldn’t think that he would be that way,” said Johnson, who grew up next door to the Fittses. “I am shocked.”

Standing with two homicide detectives outside Prince George’s police headquarters in Palmer Park, William Fitts looked very much the 15-year-old boy Wednesday. He wore a baggy T-shirt and cargo pants, and he had a little hair on his chin.

He looked down when asked by a television news reporter for comment.

A man outside the Fitts house, who appeared upset, declined to comment on the arrest.

Staff researcher Jennifer Jenkins contributed to this report.