Police rushed to the split-level corner house in Northeast Washington’s Brookland neighborhood responding to an urgent call — a man being beaten by his grandson.

Authorities said the first officer to arrive stepped inside the front door and was ambushed as he spoke with an occupant. The grandson stabbed the officer from behind, police said, striking him at least twice in the head and neck before the officer drew his handgun and fired a single, lethal shot into the assailant’s chest.

“The officer never saw him,” D.C. Police Chief Cathy L. Lanier told reporters at a news conference, describing the deadly altercation that erupted in an instant about 6:45 a.m. Friday. “He’s a very, very lucky guy to survive the attack.”

Lanier said that while being stabbed, the officer — a 10-year veteran assigned to the 5th District — had turned to confront the man and tried to shield himself from the blows. Several defensive cuts on his hands indicated that the officer may have tried to disarm his attacker, Lanier said.

Authorities did not identify the police officer, although Lanier said his wife also works on the force. She said the officer required numerous staples and stitches to close his wounds, and was released from the hospital.

Detectives and the dead man’s family were still trying to learn what might have set off 28-year-old Justin Lionel Turner, who lived off and on at the house in the 3500 block of 17th Street NE. Police and Turner’s grandfather gave slightly different accounts of what happened, and neither version explains the reason for the morning’s violence.

His grandfather, 74-year-old William Turner, said in a telephone interview that Justin Turner and his mother lived in the house he shared with his wife but left about two months ago. He showed up again suddenly early Friday morning.

William Turner, a retired stonemason, said he opened the door and welcomed his grandson home.

“We spoke. I was glad to see him,” Turner said. “I don’t know where he had gone. I thought he came home to pick up some of his stuff.”

About 20 minutes later, Turner said, his grandson suddenly became violent.

“For some reason, he got a little upset,” Turner said. “He jumped on me, hit me and put me down on the floor. He hit me three times while I was down on the floor.” He said the two had not argued and that they had never had trouble before. Turner declined to answer personal questions about his grandson.

William Turner said he managed to get up and go upstairs as his grandson went outside and his wife called police. It’s unclear when and how the younger man apparently got back inside.

Turner said he went downstairs after hearing the gunshot to see his grandson’s body on the floor next to the front door, his wife kneeling over it. He said that he did not see the shooting and that his wife was taken to the hospital to be treated for high blood pressure.

But a senior police official with knowledge of the incident said investigators were told there had been an argument inside the house Friday morning. That version had Justin Turner entering the house about 6 a.m. and using a punching bag in the basement, making enough noise to wake everyone in the house.

The police official said William Turner confronted his grandson, sparking the argument.

“Grandpa went to quiet him down,” said the official, who spoke on the condition he not be named because he was not authorized to discuss the case publicly.

He said the wounded officer rushed into the house on instinct and adrenaline, “and before he knew it, he was under attack.”

Mike DeBonis and Jennifer Jenkins contributed to this report.