Sixteen-year-old William Fitts was sentenced to 60 years in prison after being convicted of fatally stabbing a 92-year-old neighbor. (Matt Zapotosky/THE POST)

A Forestville teenager convicted of fatally stabbing a 92-year-old neighbor who had treated him like a grandson was sentenced to 60 years in prison Friday.

William Fitts, 16, pleaded guilty in April to second-degree murder and related charges in the August 2011 killing of Thelma Steele. Prosecutors said that Fitts stabbed Steele several times with a pocketknife after the two argued about family issues and that he left her for dead on the couch in her home. Her body was discovered several days later, after Fitts’s family became concerned that mail was piling up at the elderly neighbor’s house.

Wendy Tucker, Steele’s granddaughter, read a tearful statement at the hearing saying Steele probably opened the door for Fitts that day because “she thought he needed help.”

Tucker said her grandmother was so fond of Fitts that she had just as many pictures of him in her home as she did of her grandchildren. And when family members tried to persuade her to move after a February 2011 break-in, Steele said that “she felt safe because the Fittses lived across the street, and they would make sure she was okay,” her granddaughter recalled.

“She made his world a better place to be, and this is the thanks she got from him,” Tucker said.

Fitts, dressed in a green jail jumpsuit, shook his head to signal no, when Judge Maureen Lamasney asked if he had anything to say. His family rushed out of the courtroom after the hearing.

The crime shocked the neighborhood where Fitts and Steele lived. Family members on both sides have said the two were close.

Steele, a secretary at a District bank who had retired long ago, watched Fitts grow up from the window of her home on East Avenue and bought him toys and gadgets for Christmas and his birthday, family members said. Fitts, who was adopted from a family in Brazil when he was 3 weeks old, would often stop by his elderly neighbor’s house with fruit baskets, cookies and tea, they said.

But one day last summer, prosecutors said, Fitts, then 15, stabbed Steele several times with a pocketknife. He told detectives that the two had argued about family issues and that he returned later to take Steele’s purse, cash and credit card.

Homicide detectives began investigating Fitts’s possible involvement in the crime after he used Steele’s credit card to try to buy a marijuana substitute online, prosecutors and law enforcement sources said.

The 60-year sentence was part of a plea agreement. Prince George’s Deputy State’s Attorney Tara Harrison said after the hearing that prosecutors did not have enough evidence to substantiate a first-degree murder charge and that the agreed-upon sentence ensured that“justice was done.”

Tucker said that although she prayed for Fitts’s salvation, she felt he could not be rehabilitated. She vowed to attend each of his parole hearings and fight his release.

“To be capable of doing what he did was nothing but pure evil,” Tucker said.