Inside a home on 14th Avenue Northeast in Seattle, Washington, a 5-year-old boy with “behavioral problems” was allegedly locked inside a wooden box built by his father – often overnight -- for years.

>> Read more trending news

According to documents filed in King County Superior Court, the complaint “originated from a Girl Scout leader, who knows a member of the involved family and became aware of the alleged maltreatment.”

An investigator with the Washington Department of Social and Health Services/Child Protection Services then interviewed the boy and his two older siblings while they were at school, “independent of their parents.”

During that interview, the children reportedly revealed their father, 50-year-old Darin Kenneth Nelson, routinely locked his 5-year-old son "in the box when he is acting out."

The incident report, written by Kyle Kizzier of the Seattle Police Department, reveals “Darin Nelson has been imprisoning (the child) in this crate for about two years, since (the child) was three years of age" "as a means of managing 'behavioral problems.'"

Nelson allegedly admitted to a family member to taking "pictures of the box with his phone to send to potential counselors to see if this tactic was a good idea to deal "with his son's "behavior issues."

That same family member “confirmed the existence of the box” to police, and told Nelson and his wife “of her objections to the practice, but the practice has continued.”

When detectives later entered the home with a search warrant, "no box matching the description was found,” according to Kizzier’s report.

Senior Deputy Prosecuting Attorney Emily Petersen alleges in charging documents that Nelson "made other changes to the child's bedroom in order to cover up his crimes.”

In addition to unlawful Imprisonment – domestic violence, Nelson is also charged with tampering with physical evidence. Both charges are felonies.

Even though no box was found, SPD detectives did locate several small notebooks with Nelson's name on them, in which the following was allegedly written:

-November 30th: "CPS secretly interviewed (all three children's initials) @ school. Tipped off by DD."
-December 1st: "Cop shows up and asks to see" alleged victim.
-December 3rd -- "call lawyer"

KIRO-TV went to Nelson's home north of Northgate on Thursday, seeking comment about his arrest and the criminal charges filed against him. An adult who answered the door said Nelson had not lived at the home "for some time" but that he would forward the request for comment. No comment from either Nelson or his attorney has been received.

The children’s mother has not been arrested. However, documents allege it is “improbable that she was unaware of Darin Nelson’s imprisonment” of the couple’s 5-year-old son.