The biological parents of a missing Oregon boy say the 7-year-old's stepmother hasn't responded to their public pleas to fully cooperate with investigators.

Kyron Horman's father, mother and stepfather said in an e-mail to news organizations late Monday that they believe Terri Moulton Horman still hasn't resumed speaking to law enforcement.

In a statement released Thursday, Kyron's mother and father asked Terri Horman to fully cooperate with investigators

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

Kyron's father, mother and stepfather said in Monday's e-mail that they've all taken polygraph tests and would continue meeting with investigators.

A former landscaper for the boy's family told police that Terri Hormon offered him a large sum of money in exchange for killing her husband, Oregonlive.com reported Wednesday. 

The landscaper reportedly claimed that the woman discussed the so-called murder-for-hire plot with him six months before Kyron vanished. 

When detectives revealed the plan to Kyron's father, Kaine Horman, last week, he filed for a restraining order against his wife, the website reported. 

Click here to read more on this story from Fox 12 Oregon 

The Associated Press contributed to this report