The hunt for a registered sex offender suspected of killing his ex-girlfriend and her son intensified Thursday after police raided but failed to find anyone inside a home in western Missouri, where the man was charged last month for allegedly keeping the woman confined in a wooden box.

Sandra Kay Sutton, 46, and her 17-year-old son, Zachary Wade Sutton, were found dead early Thursday morning in a relative's home in Clinton. 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 with kidnapping three weeks ago. Investigators allege he kept Sandra Sutton in a wooden box off-and-on for four months at the same home they searched Thursday in Sedalia, about 45 miles northeast of Clinton.

Horn, who served time in prison for a kidnapping and sexual attack, disappeared before police could arrest him on the kidnapping charge. Lynch said Horn is "absolutely" a suspect in the deaths.

Sedalia police cordoned off his Sedalia home, where they thought Horn may have been hiding, and cleared two unoccupied homes nearby. They surrounded the home for hours, calling to Horn to come out and deploying a robot to search the area, but officers found no one once they went inside, said Sgt. Bill Lowe of the Missouri State Highway Patrol.

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

Lynch warned that Horn was an "extremely dangerous, violent person," and that anyone providing him shelter could face charges if they were aware of the pending kidnapping charges. He said Clinton authorities were unaware Sutton was staying in the area, and that court documents don't indicate she sought any protective orders again Horn.

Sutton told police in late April that 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 structure, 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 in 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 unlawfully kidnapping and abducting 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. He was still under federal supervision, said Chad Lamar, spokesman for the U.S. Attorney for the Northern District of Mississippi.

"There has been an effort to locate him on the local, state and federal level," Lamar said Thursday.

Horn's attorney in the Mississippi case declined comment Friday.