Prince George's County Police Homicide Detectives Anthony Schartner, left, and Detective Michael Ebaugh escort 15-year-old murder suspect William R. Fitts to their car in August 2011 after Fitts’s arrest in the murder of his neighbor. (Matt Zapotosky/THE WASHINGTON POST)

A Forestville teenager pleaded guilty Tuesday to fatally stabbing a 92-year-old neighbor who had acted like a grandmother to him and then making off with her credit card and cash last summer, authorities said.

William Fitts, 16, pleaded guilty to second-degree murder, first-degree burglary and theft in the killing in August of Thelma Steele, authorities said.

Prosecutors said that Fitts stabbed Steele several times with a pocketknife after the two argued about family issues and that he then left her for dead on the couch in her home. Her body was not discovered for several days, after Fitts’s family became concerned that mail was piling up at their elderly neighbor’s house.

The crime shocked the neighborhood near Forestville and District Heights where Fitts and Steele lived. Family members on both sides said the two were close. Steele, they said, watched Fitts grow up from the window of her home on East Avenue and often bought him toys and gadgets for Christmas and his birthday.

For his part, Fitts would often stop by Steele’s house with fruit baskets, cookies and tea, family members said.

But one day in August, prosecutors said, Fitts stabbed Steele several times with a pocketknife. He told detectives that the two had argued about family issues and that he returned sometime later to take Steele’s purse, cash and credit card.

Homicide detectives began investigating Fitts’s involvement in the crime after he used Steele’s credit card to try to purchase a marijuana substitute online, prosecutors and law enforcement sources said. They later found the knife used to stab Steele and clothing worn during the attack, prosecutors said.

In a statement, Prince George’s County State’s Attorney Angela D. Alsobrooks said she was “pleased” with Fitts’s plea, although she was disheartened that a person so young would commit such a heinous crime. Fitts was 15 at the time of the killing.

Fitts is scheduled to be sentenced Sept. 7. Prosecutors said in a statement that they expect he will receive a sentence of 60 years in prison.