A 25-year-old male caregiver accused of sexually abusing a teenage girl at a Minnetonka child treatment center in December should have been barred from working there in the first place, records show.

The nonprofit operator of the center, Plymouth-based Nexus, also runs other child treatment facilities in the state where sexual misconduct occurred, state investigators have found. In three other instances since 2012, Nexus treatment centers in Austin and Onamia have been cited for allowing vulnerable children to engage in inappropriate sexual contact.

In addition, the company did not always comply with the state's requirements for conducting background checks of its staff, records show.

The male caregiver at the center of the alleged abuse at the 12-bed facility in Minnetonka, known as Nexus Glen Lake, had previously been placed on a statewide "exclusion list" in July. He was sanctioned for improper billing of the state's publicly funded health insurance program, known as Medical Assistance, state officials said.

Under state rules, this should have barred him from working at the children's treatment center. Yet he was hired there four months after he was placed on the list, according to state records and Nexus.

Patient advocates say the case highlights a troubling gap in the state's system for checking the backgrounds of tens of thousands of workers who care for troubled children and other vulnerable populations. Agencies have no automatic way of knowing when a caregiver has been placed on a state exclusion list, and often do not check.

An ambitious effort to close this gap, through a new electronic background check system, has stretched on for nearly two years and will not be fully complete until late fall.

"The tragedy here is this horrific assault was entirely preventable," said Roberta Opheim, state ombudsman for mental health and developmental disabilities.

A spokesman for Nexus, which treats about 400 boys and girls at its residential treatment centers in Minnesota and Illinois, said it has fired the caregiver, but did not know he was on a state list of excluded providers. The issue did not come up on the company's background check of the staff person, said Jack Ewing, vice president at Nexus.

The man was hired in November as a youth care professional, and only began unsupervised work with clients after Nexus received a preliminary authorization from the Department of Human Services (DHS), he said.

"Nexus' priority is always to protect the safety of the clients in our care," Ewing said. "However, because of the nature of our work, and the histories and trauma of the clients we treat, these types of incidents sometimes do infrequently occur despite all best efforts."

According to a DHS report, a girl with a history of being sexually, physically and emotionally abused reported being raped the night of Dec. 22, 2015. The girl said that a male staff person entered her room, exposed his genitals, fondled her breasts, and then "anally penetrated" her. Staff later found the girl in her room with the lights off and a blanket around her waist, "crying, stuttering and shaking," the report said.

Before that night, staff reported other incidents in which the man had touched the girl inappropriately, including one incident in which he slapped her buttocks; even so, the staff did nothing to discourage this behavior, the report said.

The DHS determined that there was a "preponderance of evidence" that the male staff person at the Minnetonka facility had sexual contact with the girl. Despite the state's findings, the alleged assailant was never arrested, and the Hennepin County attorney's office decided not to bring charges against the man. A sexual assault exam and the DNA evidence collected neither confirmed nor discounted that the assault occurred, the state report said.

Hennepin County Attorney Mike Freeman said he was "horrified" by the girl's allegations, adding that "folks are not supposed to be treated that way." Yet, Freeman said his office found "no physical evidence whatsoever" that would corroborate the girl's account. "It is a very rare case that we take when the only witness to the alleged bad conduct was the victim," he said. "It's impossible to prove these cases otherwise."

Reached Thursday at his home in Coon Rapids, the former caregiver denied the sexual abuse allegations and said he did not know he was on a list of excluded providers. "I'm innocent," he said. "I didn't do what they say I did, and I will be appealing through the proper channels." The Star Tribune is not identifying the man because he has not been charged.

Jeanne Ronayne, executive director of the Minnesota Coalition Against Sexual Assault, called the county attorney's decision "regrettable," arguing that a lack of physical evidence should not rule out criminal charges. In this case, she noted, state investigators had eyewitness accounts of the man touching the girl inappropriately before the alleged assault. "I certainly see quite a bit of evidence that could be used in a criminal case — more evidence than in other cases of sexual assault that are successfully prosecuted," Ronayne said.

This marks the latest in a string of maltreatment cases at Nexus treatment centers in Minnesota. In March, the Nexus center in Onamia, known as the Mille Lacs Academy, was cited for neglect after two young residents, ages 15 and 17, admitted to engaging in sexual contact; one of the teens filled a bed with stuffed animals, then covered it with a blanket to avoid being detected. In other incidents, children at Nexus facilities were found inappropriately touching each other's genitals — both during a van trip and while in a storage shed during an outside activity, regulators found.

Nexus, which reported $44 million in revenue in 2014, was also cited last month for failing to submit background study requests to DHS for two staff persons, resulting in a $400 fine.

Staff researcher John Wareham contributed to this report.