The 20-year-old son of a woman found dead this week in a St. Paul house fire was charged Friday with strangling his mother and setting the duplex where she lived ablaze with matches and lighter fluid.

Michael Ray Caldwell, of St. Paul, was charged with second-degree murder after he confessed to the slaying at United Hospital after going there Friday for a psychological evaluation. According to a criminal complaint, an emergency room social worker called police who arrested him at the hospital.

Caldwell's mother, Lorine Mary Johnson, 49, was found dead in a first-floor bedroom early Thursday by investigators after neighbors reported a fire at a duplex on the 900 block of Carroll Avenue.

On Friday, Caldwell admitted to police that he had argued with his mother about insurance money before he strangled her and lit the fire.

Johnson and her boyfriend had flown to Kansas City on Wednesday to get Caldwell from a mental institution. About two weeks before, Caldwell had left on a bus to go to school in Oklahoma, but when Caldwell didn't arrive at the school, Johnson reported him as a missing person. Caldwell called and told his mother that he had been kicked off the bus and arrested in Kansas City. Caldwell had been in a mental health facility, but Johnson wanted him home, so she signed him out against a doctor's advice, according to the criminal complaint.

During the car ride back from the airport, Caldwell's behavior had been odd, the boyfriend told police. He said he knew of two other times Caldwell had assaulted his mother.

Caldwell told police that he knew killing his mother was wrong, the complaint said.

Nicole Norfleet • 612-673-4495 Twitter: @stribnorfleet