Robert Sparks told a 911 operator, police investigators and television news reporters that his wife had been poisoning him. So he killed her and his stepsons, and then raped his adolescent stepdaughters, according to a videotaped confession he gave to police.

Robert Sparks

"I told her, 'Look, if I find out you're putting poison in my food, I'll kill you,' " Mr. Sparks, 34, told Dallas police investigators.

That belief justified the fatal stabbing of Chare Agnew and her sons, Raeqwon Agnew, 10, and Harold Sublet, 9, on Sept. 15, 2007, according to Mr. Sparks.

The gory details of the slayings began to unfold Tuesday during the opening day of Mr. Sparks' capital murder trial in a Dallas County courtroom.

Mr. Sparks told police that he had been recording the kids and that on a tape there was proof that they had been poisoning him. Raeqwon also told Mr. Sparks that he'd seen his mother putting something in her husband's food, Mr. Sparks said.

"Basically, they trying to put this all on me," Mr. Sparks told police after his arrest, three days after the stabbing deaths. "I've been poisoned."

Dallas County prosecutor Andy Beach used his opening statement to prepare jurors for the testimony they would hear Tuesday. He said Mr. Sparks started his killing spree with the only other adult in the couple's southeast Dallas home.

"He looked at his sleeping wife and he put his hand over her mouth," Mr. Beach said. "He put his knee in her stomach for leverage and in his own words, 'dug in' " with the knife.

Mr. Sparks told police that a voice told him to kill them all. And once he started, Mr. Sparks said, he felt nothing.

"I was blank. It wasn't no feeling anything. It was just, get it over with," said Mr. Sparks.

After stabbing Ms. Agnew, Mr. Sparks picked up the boys one at a time from their bunk beds. With the boys still wrapped in their blankets, he took them to the kitchen, Mr. Sparks said. First, he stabbed Harold 45 to 50 times. Then he went back to the bedroom to get Raeqwon, placed him on the kitchen floor and repeatedly stabbed him.

"I wanted it to be quick, but he fought," Mr. Sparks said of the older boy.

Afterward, Mr. Sparks said, he woke his stepdaughters – who were 12 and 14 – and raped them. He allowed them to live, he said, because he wanted them to suffer. He bound their feet and hands and locked them in a bedroom closet. Mr. Sparks told police that he put some water and a fan in the closet so the girls wouldn't get too hot.

If convicted, Mr. Sparks faces either death or life in prison with no opportunity for parole.