PORTLAND, Ore. -- A newly released court document shows the father of a 7-year-old Portland boy missing for more than a month told a judge he believes the boy's stepmother is involved in the disappearance.

Kaine Horman also said in his request for a restraining order against Terri Moulton Horman that investigators told him that he had been the target of a murder plot by his wife in late 2009.

The decision to obtain a restraining order late last month "was based on a lot of information provided by law enforcement," Kaine Horman said Thursday. He declined to elaborate.

Before little Kyron Horman's disappearance on June 4, Kaine Horman said his relationship with his wife was "a nice clean slate, as far as I can tell."

The court documents released Thursday appeared to confirm a report Sunday by The Oregonian newspaper about allegations that Terri Horman had sought to have someone kill her husband several months before the boy's disappearance.

Kaine Horman said in an interview with The Associated Press that Terri Horman began to suffer from postpartum depression after the birth of their daughter Kiara in 2008. He said his wife didn't tell him she was depressed, but "vented" to friends, including Desiree Young, Kyron's mother.

"I think with the birth of our daughter, she definitely changed emotionally," Kaine Horman said.

"She did have some postpartum depression after the pregnancy and she had some very strong difficulties with that."

Kaine Horman said his wife has twice failed a polygraph test during the investigation into Kyron's disappearance. She shared the news of the failed tests with "a room full of people," he said Thursday.

"She would vent about the failure, talk about what she failed on, go into great detail about how she felt about it, and that happened multiple times," he said.

Police have said Terri Horman was the last known person to see the boy. She told police she left him at his school, about two miles away from their home, after an early morning science fair.

Young said Thursday at a news conference that she suspected Terri Horman's involvement soon after the boy was reported missing.

"I've known her a long time," Young said. "I know she's lying."

Young, who lives in Medford with her husband, said Terri Horman's explanation after the boy's disappearance didn't add up.

"There are just some things in there that didn't sit well with me, and it just didn't sound right," Young told The AP. "I'm very familiar with that school and there were just some things that I knew couldn't be correct."

Investigators have not named Terri Horman as a suspect or a person of interest, but in recent weeks they have focused their investigation on her. They distributed fliers asking for information on her whereabouts the day the boy vanished.

A call to Terri Horman's lawyer was not immediately returned Thursday.

A former teacher and ex-bodybuilder, Terri Horman had become increasingly isolated from family members.

Late last month, Kaine Horman moved out of the couple's home in the countryside west of downtown Portland, filed for divorce and got the restraining order against his wife -- including a provision that she not have access to firearms or ammunition.

The restraining order forbids her from coming near her husband or their 19-month-old daughter. Now his lawyer has filed a motion to amend it to force Terri Horman to vacate the family home.

The restraining order was unsealed by a judge on Thursday, following requests by a coalition of news organizations, including The Associated Press, that it be made public.

The order contained the first publicly released suspicion by Kaine Horman that his wife was connected to Kyron Horman's disappearance.

"I believe respondent is involved in the disappearance of my son Kyron who has been missing since June 4, 2010," Kaine Horman's petition reads. "I also recently learned that respondent attempted to hire someone to murder me. The police have provided me with probable cause to believe the above two statements to be true."

His statement contained no details on how Terri Horman might be involved in the boy's disappearance.

The sheriff's office has refused to confirm details of the alleged murder-for-hire plot and did so again Thursday. In a statement following the parents' news conference, the sheriff's office said "the information released during these conferences did NOT come from the Multnomah County sheriff's office."

Kaine and Terri Horman were married in April 2007. Terri Horman also has a teenage son from the first of two previous marriages.

The investigation has become one of the most intense in recent Oregon history, and has attracted national attention.

At the start of the search, Terri Horman organized friends to pass out fliers. She also appeared at two early news conferences with other members of the family.

The boy's parents on Thursday reiterated their contention that Terri Horman isn't cooperating with investigators -- despite assurances from the sheriff that she is -- and continued to call on her to tell investigators the truth.

"She needs to do the right thing, being honest," Young said.

"Whatever that truth is," Kaine Horman added.