A Kansas couple who failed to report their son's disappearance nearly a decade ago are considered "people of interest" in the case as authorities expand their search nationwide, a sheriff said Monday.

Butler County Sheriff Craig Murphy told reporters no charges would be filed while investigators focus on finding Adam Herrman, who was 11 when he disappeared in 1999 from a mobile home park in Towanda where he lived with Doug and Valerie Herrman.

Adam had been in foster care and was adopted by the Herrmans when he was 2 1/2 years old. He was born June 8, 1987, and would now be 21 years old.

"We are not sure what happened to Adam Herrman," Murphy said. "We are working it as if it is a death — but we are not leaning one way or the other."

Asked why the parents have not been arrested, Murphy replied: "`We are not ready' is an honest and upfront answer."

The family's attorney, Warner Eisenbise, said his clients had nothing to do with harming the child.

"He was a problem child. He ran away frequently to the point of exasperation," Eisenbise said. "My clients feel very guilty that the last time he left they didn't make an attempt to locate him. Every other time, the police were called or he wandered back. They assumed he found one of his siblings or went back to his biological parents."

Murphy said investigators have not confirmed whether Adam had a history of running away. The family has cooperated with investigators, he said.

The Herrmans now live in the Wichita suburb of Derby, in neighboring Sedgwick County.

Eisenbise said they want to speak out but that he is advising them not to talk to the media.

"They want to say they are innocent of any wrongdoing other than not reporting the child missing, which is an offense in Kansas as you know," he said. "The issue of a possible homicide is what I am troubled, with because they did not do it."

Murphy's office did not receive a missing persons report until contacted recntly by the Sedgwick County Exploited and Missing Children's Unit. He declined to say who tipped them off.

But Eisenbise said his clients believe the boy's biological sister or some other relative went to police years later and started the investigation.

Murphy said a search of the now empty lot where the family's mobile home once stood gave investigators one answer they sought, but he did not elaborate other than to say no human remains were found.

The boy was being homeschooled at the time he disappeared, Eisenbise said.

Investigators have not found any confirmed data on Adam's whereabouts since 1999.

Authorities are asking anyone with information about the case to call the Butler County sheriff's office.

"If Adam Herrman is alive out there — and he would see this — I would ask him to contact us immediately," Murphy said.