Updated

Prosecutors filed charges Tuesday against an Oklahoma couple in the 2006 disappearance of their 9-year-old nephew, alleging the couple beat the boy to death and coerced his brother to repeat a rehearsed story to authorities.

Rex and Rebecca Clark, who were arrested April 8 in connection with Colton Clark's disappearance, were each charged with first-degree murder, intimidation of a witness, conspiracy to commit a felony, two counts of child neglect and four counts of child abuse. The charges came on the eve of the 10-year anniversary of the day Colton, whose body has not been found, was reported missing.

The Clarks, who are in custody, maintain their innocence on all counts, defense attorney Robert Butler said Tuesday.

"They adopted these children and then inherited a horrible situation," Butler said. He declined to comment further on the charges.

The Seminole County Sheriff's Office has been searching the Clarks' property in Seminole since the arrests, but investigators have not found a body, investigator David Hanson said Tuesday evening. They were waiting for the ground to dry out from recent rains before continuing the search, Hanson said.

Authorities obtained a warrant to search the property after reopening the case late last year and conducting a series of interviews, including with Colton's older brother.

In an arrest affidavit made public last week, authorities say the brother told investigators that Rex and Rebecca Clark would accuse the boys of stealing and then beat them, sometimes using objects like broomsticks, electric cords and cattle prods.

One night, the brother told investigators, he walked into a room and saw Colton lying motionless on the couch, face bruised from a recent beating. It was the last time he would see his brother before he disappeared, he told investigators, according to the affidavit.

After Colton disappeared, Rex and Rebecca Clark told the brother to repeat a rehearsed story to anyone who asked about the missing boy, the affidavit alleges. The brother said he feared for his life.

The Clarks' next date in court is a preliminary hearing in June, although that date could change depending on a judge's availability, said Paul Smith, Seminole County first assistant district attorney. Smith said he could not comment on whether evidence not listed in the affidavit, such as what is being gathered at the property, might have influenced the charges.

Butler says he is beginning an investigation on behalf of his clients.

Colton and his older brother lived with their aunt and uncle after their parents terminated their rights to the boys. Colton was reported missing on April 20, 2006, prompting a 2,500-acre search aided by helicopters and dogs. The Clarks said he wouldn't attend a counseling session, and investigators said at first the boy may have run away, since his backpack was gone.