The stranger ran to the back door of Thomas Sonnenberg’s north Minneapolis house, screaming “Let me in! Somebody’s going to kill me!”

Sonnenberg, a 69-year-old retired technician known as a good Samaritan on his block, stepped outside, looked around and ushered the man inside, then was careful to lock the deadbolt.

At the stranger’s behest, he called 911. Moments later, Sonnenberg lay on the floor, with a bullet to the head, and the man he had sought to shelter from harm was attacking Sonnenberg’s 68-year-old wife.

Police arrived in time to interrupt the midday Friday assault in the small house in the 3700 block of Aldrich Avenue N., but they could do nothing for Sonnenberg, who died of the gunshot wound.

The home invasion and killing, as recounted Saturday by one of the Sonnenbergs’ daughters and by police reports, was the fulfillment of the family’s fears about the dangers the retired couple faced in their longtime home on the ­border of the Camden-McKinley neighborhoods.

Rachel Sonnenberg Baufield said Saturday that for years, she and her two sisters had begged their parents to move away from what they saw as an increasingly risky block.

But the Sonnenbergs, who had endured burglary and vandals on their property, felt that they had no choice but to stay — they were trapped by an ­underwater mortgage. Still, Baufield said, her father had remained kindhearted, unable to turn away people who cried on his doorstep, including women who claimed they needed to leave town to escape abusive mates.

“My dad has helped this community,” she said. “There have been other people who have come to my dad and begged for help, and my dad gave them money because he didn’t want people hurting. And this is the repayment that he gets.”

Police arrested a 20-year-old felon with a ­violent history in the Sonnenberg slaying.

Devon Derrick Parker of Minneapolis, who was on probation for a felony assault in 2011, was in the Hennepin County jail on Saturday on suspicion of murder, awaiting formal charges. Police have not yet disclosed a motive, but family members believe the attack was planned as a robbery.

‘He went on a rage’

Thomas Sonnenberg retired several years ago from his longtime job as a test technician for FMC, an industrial chemical company.

His wife, Elaine, was recovering at a family member’s home Saturday night after being treated for attempted strangulation and other trauma. Police were searching for the gunman’s girlfriend, who may have witnessed him entering the home but apparently was not part of the plot, police said.

Baufield said the family wants justice for their father, who had installed keyed deadbolts on his doors, in the hope of keeping the house safe. In the end, the deadbolts trapped the killer; the suspect didn’t know which key to use to get out, Baufield said.

“After the 911 call was made, he shot my dad in the head,’’ she said. “And then he went on a rage because he was locked in the house, and threw stuff at my mother and assaulted my mother. He couldn’t get out of the house to get away.”

Sonnenberg called 911 at 11:44 a.m., records showed. During the call, police said, the suspect grabbed the phone, falsely claiming that “The Circle of Discipline” was trying to kill his brother at a house a block away.

A nonprofit boxing organization that helps guide young people at risk in ­Minneapolis uses that name, though it’s uncertain if the suspect was referring to that same group.

Elaine Sonnenberg had been in another room and went to the kitchen to see what was going on.

“She came to see what had happened and watched him get shot,” Baufield said. “And then it escalated from there when [the suspect] couldn’t get out. He threw keys. He threw whatever he could find at her. And then he threw her around,” the daughter said.

“He drug her upstairs to her bedroom with the gun to her forehead, placing the gun in the same spot on her head as where he’d shot my father, between his eyes.”

Police responding to the call were detoured briefly because the suspect gave them an ­incorrect address. Five minutes after being dispatched, police arrived at the incorrect address in the 3700 block of Bryant Avenue, only to learn there was no house there. They arrived at the Sonnenbergs’ home at 11:55 a.m., said police spokesman John Elder.

Police knocked on the doors, then broke into the house and arrested Parker.

Baufield said her mother “is holding on” but will not be returning to the house. The family has already begun the process of moving out the couple’s possessions.

Baufield said her father, an Army veteran, was a private man who dearly loved his eight grandchildren.

“He’s been our rock, so this is very difficult for all of us,” she said.