PHOENIX -- Federal authorities allege that a former youth care worker at a Phoenix-area facility for immigrant youths sexually abused eight teenage boys, one of several cases brought to light in recent weeks as thousands of immigrant children remain detained around the country. Court documents show Levian Pacheco faces several charges stemming from incidents that allegedly took place between August 2016 and July 2017 at a Southwest Key facility in Mesa.

The case was first reported by ProPublica. Authorities charge that Pacheco performed sex acts on two boys and touched six others, all between ages 15 to 17 at the time.

"One of the boys decided to speak up about what had happened to him, and then it was kind of a snowfall where another boy — the roommate — spoke up that it also happened to him," Michael Grabell, one of the ProPublica reporters who uncovered the details of abuse at the Southwest Key shelter, told CBS News.

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Court documents also state that Pacheco is HIV-positive and that some of the teens opted to be tested for the virus.

Pacheco has pleaded not guilty and denied the allegations. His federal public defender has said in court documents that the government's allegations of the crimes committed by Pacheco include an "extraordinarily broad range of dates and lack of specificity."

"We are looking forward to defending Mr. Pacheco in court," attorney Benjamin Good said in an email to The Associated Press.

Grabell told CBS News that it's common for victims of abuse to be afraid to come forward. "Especially in this population, there is this fear that if you come forward and report abuse, it might affect your immigration case," he said.

The Department of Health and Human Services told CBS News that "any allegation of abuse of neglect is taken seriously... and appropriate action is taken."

The revelations Thursday came one day after police said another worker at a different Southwest Key facility, Fernando Magaz Negrete, is suspected of molesting a 14-year-old immigrant girl.

Authorities say Magaz Negrete kissed and fondled the girl in her bedroom on June 27 in an encounter witnessed by the girl's 16-year-old roommate. They said surveillance video shows Magaz Negrete, 32, approaching the bedroom several times throughout the night, though the bedroom itself was outside the camera's view.

Southwest Key fired Magaz Negrete. It was not clear if he is represented by an attorney.

The Texas-based nonprofit organization, the largest contracted by the U.S. government to house immigrant children, has come under scrutiny for its practices at its various shelters around the Southwest. Many of the over 2,000 children who were separated from their parents after crossing the border illegally have been held at Southwest Key facilities.

Following backlash over the Trump administration's child separation policy, officials defended the use of the detention facilities as safe spaces, CBS News correspondent Don Dahler reports.

"These shelters were never intended to be long-term group homes," Grabell told CBS News, "and we've heard from various officials, various psychologists that the longer the kids are in the shelter the more time there is for inappropriate relationships to develop."

The victims in the case against Pacheco were all unaccompanied minors and it's not clear if they had been separated from their parents or traveled to the U.S. alone, although the government was not widely separating families at the time of the incidents.

Southwest Key spokesman Jeff Eller said in an email that Pacheco was immediately suspended and police were called when allegations surfaced.

He said Southwest Key and the Health and Human Services department, which is in charge of caring for immigrant youths, take every incident seriously "and there is a never ending process to improve care and standards."

ProPublica published a report in July saying police responded to at least 125 calls reporting sex offenses at shelters that primarily serve immigrant children since 2014. The report detailed a 2015 incident at a Tucson, Arizona, Southwest Key facility in which authorities said an employee touched a 15-year-old Honduran boy's penis from outside his clothing. The worker was fired and later convicted of molestation.