Desiree Young called on Kyron Horman's stepmother, Terri Horman, to "do the right thing" and cooperate with investigators, if for no other reason than for the boy's half sister, 18-month-old Kiara Horman, "who loves him and misses him."

MEDFORD — The biological mother of a 7-year-old Oregon boy missing for more than a month issued a fresh appeal Tuesday to the boy's stepmother, asking her to cooperate with police.

Desiree Young called on Kyron Horman's stepmother, Terri Horman, to "do the right thing" and cooperate with investigators, if for no other reason than for the boy's half sister, 18-month-old Kiara Horman, "who loves him and misses him."

Terri Horman is the last person known to have seen Kyron before he vanished on June 4. Investigators have not named her as a suspect or a person of interest.

Her attorney, Portland defense lawyer Stephen Houze, did not immediately return calls seeking comment on Tuesday.

Multnomah County authorities have said that Horman was cooperating with the investigation. But Young, the boy's father and his stepfather said in an e-mail to news organizations late Monday that they believe Horman is not telling investigators everything she knows.

Lt. Mary Lindstrand at the sheriff's office said Tuesday that the department would not discuss details of the case.

Horman has told authorities she dropped him off at a science fair before school. He never appeared in class, and did not come home on the school bus.

Young and her husband, Medford Police Detective Tony Young, said Tuesday they wanted people in southern Oregon to know about his ties to the region, and to keep up hopes that he will be found alive.

"We love you and we miss you," his mother said. "Please do not be afraid, because the police are going to find you."

Young also revealed that the day her son disappeared, his stepmother was supposed to drive him to Eugene to turn him over to his mother and stepfather to spend the weekend in Medford.

Tony Young said he was going to take the boy on a long-delayed fishing trip with the rod the stepfather had given him for Christmas.