John W. Wilfong, 17, was arrested Wednesday morning after an all-night search to find him and a 9-year-old who also lived at the Ackman residence.
Ackman's husband discovered her body in the kitchen of their home, off Highway 51 in Patton, just south of Highway 72.
When sheriff's deputies responded to the homicide they discovered the two boys missing, along with the .22-caliber rifle that belonged in a gun cabinet in the residence and the Ackmans' gold 1999 GMC Yukon.
Ackman suffered multiple gunshot wounds to the head with a .22-caliber firearm and was pronounced dead at the scene, said Bollinger County Coroner Charles Hutchings.
After the shooting, Wilfong and the juvenile drove to Marble Hill, Mo., where they left the vehicle at the city park, according to a probable-cause statement by Cape Girardeau Detective Don Perry.
Around 5 p.m., Jerry Fox, a second cousin of Wilfong's, heard a knock on the door of his Marble Hill home. He was puzzled when he saw the two boys standing there, Fox said Wednesday.
They told Fox and his brother they needed a place to stay, not saying a word about the killing, Fox said.
Later that evening, Wilfong said he'd parked his truck in town and locked a rifle in it, Fox said.
A Bollinger County sheriff's car blocks the driveway leading to the home of Pamela Ackman off Highway 51 in Patton, Mo. Wednesday morning. Ackman was found dead Tuesday from multiple gunshot wounds to the head.
"I called the law and told them they were here," Fox said.
Fox told Wilfong that if he ran, it would only make things worse for himself, he said.
When Fox gave a statement to investigators, he learned he and his brother had actually been related to Ackman, whose maiden name was Wilfong.
The 9-year-old was transported to Cape Girardeau Juvenile Detention Center, and Wilfong was taken to the Bollinger County Jail, where he confessed to the murder, police said.
He was not charged with kidnapping because the juvenile went with him willingly, Sheriff Leo McElrath said.
Wilfong told investigators he planned Ackman's murder Tuesday while she was gone.
"They wanted out of there, they didn't want to live there anymore, and that was their solution," McElrath said.
According to the sworn statement, Wilfong and the juvenile took the rifle from the gun cabinet, and Wilfong removed the scope because he knew it didn't work properly.
The juvenile watched from his bedroom window, waiting for Ackman's vehicle to pull into the driveway and alerted Wilfong when it did, the statement said.
Wilfong told detectives he waited in the living room and when Ackman entered through the kitchen, he aimed the rifle at her head and pulled the trigger.
He thought the first shot struck her in the neck or the back of the head, so when she fell to the floor and was still moving slightly, he fired again, the statement said.
Wilfong is charged with armed criminal action and first-degree murder. He is scheduled for a first appearance in a Bollinger County court on Wednesday.
According to Brian Hauswirth, a spokesman with the Missouri Department of Social Services, Wilfong was not in a foster-care program, though Ackman was a licensed foster mother in Missouri and the 9-year-old is believed to have been a foster child.
Ackman and her husband, Mickey, were granted guardianship of Wilfong in November 2003. The Ackmans applied for guardianship of Wilfong, under protest by his biological mother. Both biological parents were deemed unfit by the court.
In 2002, Wilfong's father, Henry J. Wilfong, was charged with first-degree murder in Wayne County, Mo., and is being held at Fulton State Hospital after the court found him mentally incapable of standing trial.
McElrath said Wilfong's mother, Sonya Wilfong, was extremely upset Wednesday and believes her son could not distinguish between right and wrong.
Ackman's funeral will be at 11 a.m. Saturday at Liley Funeral Home in Patton.