A teenage mother shot and killed an intruder after a 911 operator said she was allowed to defend her infant son and herself with force.

Sarah McKinley, 18, killed Justin Martin with a single gunshot wound on New Year's Eve when he forced his way into her Blanchard, Okla. home and came at her with a long hunting knife, ABC News reports.

The deadly encounter occurred about a week after the young mom's husband died of cancer, according to TV station KOCO. Martin darkened McKinley's door on the day of her husband's funeral, several days before the shooting. He claimed he was a neighbor who wanted to say hello, but she didn't open the door.

Martin returned with an accomplice on Dec. 31, and they tried to force their way into the modest house. When McKinley heard the men trying to break in, she called 911.

She also holed up in her bedroom with a 12-gauge shotgun and a pistol, while she put a bottle in her three-month-old son's mouth.

"I've got two guns in my hand -- Is it okay to shoot him if he comes in this door?" McKinley asked the 911 operator.

"I can't tell you that you can do that, but you do what you have to do to protect your baby," the dispatcher told McKinley when she asked a second time. The call went on for 21 minutes as the men powered their way into McKinley's house.

Eventually, Martin kicked in the door and charged at her with a knife, but McKinley fired before he could injure her.

Police found Martin slumped over a couch that McKinley had used to barricade the door and pronounced him dead on the scene, TV station News 9 reports. They said McKinley's use of force was justified.

Martin's alleged accomplice, Dustin Stewart, fled when he heard the gunshot and later surrendered to police, according to The Oklahoman. Stewart was charged with burglary.

UPDATE: 6:30 pm -- Stewart now also faces murder charges, because of his participation in the botched break-in that resulted in the death of his accomplice, according to local sources.