More than half of the allegations of sexual abuse of unaccompanied migrant minors by adult staff occurred in shelters run by just three contractors — nonprofits that received federal grants totaling more than $2.5 billion over the past four years, according to USAspending.gov.

Expand chart Data: Office of Refugee Resettlement; Chart: Andrew Witherspoon/Axios

By the numbers: The federal government has received as many as 10 separate reports of alleged sexual abuse by staff at multiple migrant child shelters over the past four years, totaling 178 allegations against adult staff members, according to HHS documents given to Axios.

These staffers work at shelters for migrant children, which are operated by nonprofits and funded by the Department of Health and Human Services. HHS says it's responsible for "overseeing the infrastructure and personnel of [Office of Refugee Resettlement]-funded care provider facilities" and "ensuring compliance with ORR national care standards."

at shelters for migrant children, which are operated by nonprofits and funded by the Department of Health and Human Services. HHS says it's responsible for "overseeing the infrastructure and personnel of [Office of Refugee Resettlement]-funded care provider facilities" and "ensuring compliance with ORR national care standards." HHS declined to comment for this story.

Southwest Keys Programs (SWK) is the largest of the nonprofit contractors running shelters for migrant children. It has received $1.5 billion from government contracts over the past four years, according to USAspending.gov. During that time, HHS data showed there were 65 allegations of sexual abuse by staff at their shelters.

One allegation involved a youth care worker twice offering a minor a pair of shoes in exchange for fondling the minor's genitals. The case was reported to the Justice Department, but not investigated.

involved a youth care worker twice offering a minor a pair of shoes in exchange for fondling the minor's genitals. The case was reported to the Justice Department, but not investigated. "Any mistreatment of a child is unacceptable and violates the mission that guides our organization," Neil Nowlin, a spokesperson for SWK, told Axios. " If the authorities determine a report is credible and launch a formal investigation, we participate fully in that process."

is unacceptable and violates the mission that guides our organization," Neil Nowlin, a spokesperson for SWK, told Axios. If the authorities determine a report is credible and launch a formal investigation, we participate fully in that process." The company is also currently under investigation by the Justice Department for misuse of government funds, the New York Times reported.

by the Justice Department for misuse of government funds, the New York Times reported. SWK recently shut down two shelters in Arizona and was forced to pay a $73,000 settlement to the state health department that had been seeking to revoke the company's licenses for not providing proof that its workers had received required background checks. This came after reports that three children were physically abused by staff in one of the shelters, according to Arizona Central.

Baptist Child & Family Services (BCFS) is part of a larger organization involved in various international relief efforts, educational services, adoption, foster care and other health and human services, according to their website.

23 allegations of sexual misconduct by staff members occurred in BCFS shelters, several of them involving inappropriate relationships between adults and migrant minors, as well as at least two instances where pornographic images were allegedly offered or shown to minors.

by staff members occurred in BCFS shelters, several of them involving inappropriate relationships between adults and migrant minors, as well as at least two instances where pornographic images were allegedly offered or shown to minors. The nonprofit has received received $768 million in government funding since 2015, according to USAspending.gov.

in government funding since 2015, according to USAspending.gov. BCFS built and managed the "tent city" in Tornillo, Texas, which housed hundreds of migrant children last year. The shelter has since been closed after thousands of minors were released from HHS custody.

the "tent city" in Tornillo, Texas, which housed hundreds of migrant children last year. The shelter has since been closed after thousands of minors were released from HHS custody. "It’s also worth noting that ORR reporting criteria is extremely broad, which contributes to the high number of reports," BCFS spokesperson Krista Piferrer told Axios in a statement. "It is also common practice for BCFS Health and Human Services to 'over report' to ORR, state licensing and law enforcement, meaning we report anything that might even come close to meeting reporting criteria."

ORR reporting criteria is extremely broad, which contributes to the high number of reports," BCFS spokesperson Krista Piferrer told Axios in a statement. "It is also common practice for BCFS Health and Human Services to 'over report' to ORR, state licensing and law enforcement, meaning we report anything that might even come close to meeting reporting criteria." BCFS also noted to Axios that many of the investigations resulted in no findings and that none of its employees have ever been charged with sexual abuse of a child in their care. When there are issues, employees are immediately removed from contact with youth, the spokesperson said.

International Educational Services (IES) shelters were responsible for the third highest number of staff-related sexual abuse allegations with 14 reports filed to the Justice Department. The nonprofit was shut down in early 2018 after an audit by the HHS inspector general, according to the Brownsville Herald.

It received almost $300 million in government contracts over the past four years, but has not received any funding since April 2017, according to USAspending.gov.

in government contracts over the past four years, but has not received any funding since April 2017, according to USAspending.gov. The Brownsville Herald noted that neither IES nor the HHS Office of Refugee Resettlement has said why the nonprofit shut down.

that neither IES nor the HHS Office of Refugee Resettlement has said why the nonprofit shut down. However, a letter obtained by the Brownsville Herald through a Freedom of Information Act request said the government limited its cost reimbursements starting in November 2017 because of concerns that it didn't show enough control and accountability over federal funds.

The big picture: HHS received 4,556 complaints of sexual misconduct by minors and adults from October 2014 to July 2018, and the Department of Justice received a total of 1,303 complaints. Axios does not have information on the location of offenses by minors or adults who were not staff.

The backstory: These shelters have long worked to care for migrant youth who cross the U.S. border illegally, but without drawing much attention. President Trump's family separation policy — which separated thousands of migrant kids from their parents — put them in the spotlight.