"I can't get them out of my mind," said Dan Porter, his voice breaking, said while sitting in a sheriff's van after making his first court appearance on murder charges.
He was charged earlier in the day with two counts of first-degree murder in the shootings of his son, Sam, 7, and daughter, Lindsey, 8.
Porter, 44, already was serving a 38-year prison term for kidnapping the children to terrorize their mother, Tina Porter.
The children's bones were found in September in a wooded area near the Missouri River in nearby Sugar Creek and were identified using dental records. Sam and Lindsey had been missing since Porter picked them up from their mother's Independence home for a weekend visit in June 2004.
"I'm sorry," Porter told reporters after the brief court appearance. "No matter how sorry I am, I realize there are going to be people who never forgive me."
Asked why he did it, Porter said: "Could any man come up with an excuse for that? Is there such an excuse?"
As he was led to the van that would take him back to jail, Porter, wearing an orange prison jumpsuit and shackled at his wrists and ankles, acknowledged to reporters that he had confessed to the killings. Asked why he waited so long, Porter said: "I couldn't take it no more."
During Tuesday's hearing, a judge read the charges and not-guilty pleas were entered for Porter. A preliminary hearing was set for Dec. 19.
Porter did not have an attorney at the hearing, but public defender Timothy Burdick was assigned to him afterward. Burdick did not return phone or e-mail messages seeking comment.
According to the probable cause statement filed with the charges, the children's remains were found after Porter met with FBI agents and Missouri Department of Corrections officials on Sept. 7 and confessed that he killed his children on the day he had picked them up for the visit. The documents also say Porter led agents to the spot where the children were killed and buried.
Court documents say Porter told authorities he had been planning to kill his children since two days before their visit.
Jackson County prosecutor Jim Kanatzar said he would decide within the next month whether to seek the death penalty. He said capital punishment was being considered because of the circumstances of the crime.
"All cases involving children are horrific. This one particularly struck a chord because of the ages of the children and the fact that the defendant was their father. It's a terrible case," Kanatzar said during a news conference.
He said the children were killed with a handgun, but he could not provide additional details about the crime, including why Porter finally confessed.
"There is absolutely no explanation for actions like this," he said.
After his arrest on the kidnapping charges, Porter told authorities several stories about what he had done with his children, including that he had cut them up and that he had strangled them.
According to police, the children's remains were found in the area where the Porters met after Dan Porter had picked up the children. Police have said Dan Porter asked Tina Porter to meet him there so they could exchange vehicles.
Tina Porter, 44, has said that when she met Dan Porter, the children weren't with him. She said Dan Porter tried to get her to drive her pickup into the woods by telling her that he had stashed $50,000 there and wanted to get it. She refused.
At the time of the children's disappearance, Dan and Tina Porter were estranged. They have since divorced.
Tina Porter did not return messages seeking comment Tuesday. Van Buckley, a spokesman for the Jackson County prosecutor's office, said prosecutors have advised her not to talk to reporters.
By Heather Hollingsworth