Updated

A man has been charged with the stabbing and strangulation murder of his two stepdaughters in what police are calling a "spell gone bad" in the family's Sioux City, Iowa, home.

Lawrence Douglas Harris Sr. was charged with two counts of first-degree murder after authorities arrived at his burning home and found the girls in their second-story bedroom and blood on Harris, police said Monday.

Police Lt. Marti Reilly said Harris had been performing "some strange ritual" and told officers he cast a spell that had gone bad. Reilly said he could offer no other details on how the girls were killed.

The victims include Kendra Suing, 10, and her sister, Alysha Suing, 8, whose bodies were found Sunday afternoon. An official cause of death won't be available for several days, Reilly said.

"You're talking about people casting spells, spells gone bad," Sioux City Police Chief Joe Frisbie said at a Monday press conference, according to the Sioux City Journal. "Obviously, there is a lot more going on here than a straightforward homicide."

Click here to read the Sioux City Journal report.

The fire, which started in the basement of the home, caused only superficial damage, Reilly said. The cause of the fire remained under investigation.

Harris was the only adult at the home, and he told officers he had been the only other person there. The girls' mother was at work at the time, Reilly said.

Reilly said the family had moved to the community within the past year and that police had no prior contact with the family.

Reilly said he didn't know what, if any, weapons were found in the home.

He referred further questions to Capt. Melvin Williams, lead investigator in the case. A telephone message left for Williams on Monday afternoon was not immediately returned.

Harris was being held in the Woodbury County jail under $2 million bond.

Experts say the study of witchcraft is often misunderstood.

Professor Helen A. Berger, author of three books on witches, told the Sioux City Journal she doubted anyone claiming to have killed children while casting a spell is a true practitioner of witchcraft.

Click here to read the full Sioux City Journal report with the expert.

"This is not a group that participates normally in violence, but it is a group that doesn't have firm boundaries, which means that anyone can make a claim to be a member," she told the newspaper.

The Associated Press contributed to this report.