SEDALIA, Mo. -- Police have surrounded a home looking for the ex-boyfriend of a woman who was found dead Thursday, along with her teenage son, less than a month after the man was charged with holding her captive in a wooden box for months in western Missouri.

Sandra Kay Sutton, 46, and her son, Zachary Wade Sutton, were found dead in a relative's home in Clinton, Missouri, about 4 a.m. Thursday.

Both victims appear to have been shot overnight while Sutton's relatives were at work, Clinton police Lt. Sonny Lynch said.

Police have been searching for her former boyfriend, James Barton Horn Jr., since he was charged earlier this month with kidnapping.

Investigators allege he kept Sandra Sutton in a wooden box off and on for four months at the home they shared in Sedalia.

Horn, a registered sex offender who served time in prison for a kidnapping and sexual attack, disappeared before police could arrest him on Sutton's kidnapping allegations. Lynch said Horn is "absolutely" a suspect in the deaths.

A police officer found a car Sandra Sutton had used outside a hospital in Sedalia, Lynch said. Surveillance video from the hospital showed a man authorities believe was Horn walking away from the car and toward nearby houses at 4:15 a.m., the lieutenant said.

One of the houses, a few blocks from the hospital, was where Horn allegedly kept Sutton confined in the box. Sedalia police cordoned off that home, where they thought Horn might have been hiding.

They surrounded the home for hours, calling to Horn to come out and deploying a robot to search the area, but found no one once they went inside, said Sgt. Bill Lowe of the Missouri State Highway Patrol.

They also searched two unoccupied homes nearby.

Federal authorities are helping search for Horn, who is under the supervision of federal probation officers.

Lynch warned Horn was an "extremely dangerous, violent person," and anyone providing him shelter could face charges if he or she was aware of the pending kidnapping charges.

In late April, Sutton told police Horn had threatened her in a car with a "tire jack" after an argument in January.

When they returned home, he began building a wooden box, and she had to help with its construction, according to a probable-cause statement filed by Sedalia police.

Sutton was allowed to leave the box in the evenings when Barton returned home from work, according to court documents.

Police said the box was kept in a bedroom and contained a bucket full of urine and feces.

The box, which investigators said was 100 inches long, 48 inches wide and 52 inches tall, also contained several layers of insulation, padding and sleeping bags and a small air hole.

Sutton escaped April 30 and ran to the home of a neighbor who called police, but Horn was gone by the time police arrived.

A neighbor, Roeanna Wright, said she saw Sutton crouch down after running from the couple's home in April. Her husband, Lloyd Wright, said she looked panicked, adding: "She was crouched down like she was trying to hide when she ran."

Horn served about three years prison in Tennessee in the early 1990s in connection with a kidnapping and sexual attack. Records also show he pleaded guilty in 1997 in Mississippi to kidnapping his estranged wife.

He was sentenced in that case to 12 years and 11 months in prison, plus five years supervised release.

He was released from custody in December 2011, and his probation jurisdiction was transferred in 2012 to Missouri, according to online court records.

Sudekum reported from Kansas City, Missouri.