An advocacy group that represents whistleblowers sent a scathing letter Thursday to the House and Senate committees overseeing U.S. Immigration and Customs Enforcement, calling on the congressional bodies to investigate abuses by ICE and the executive branch’s failure to hold the agency accountable.

The group, Government Accountability Project, cited a report from the Department of Homeland Security’s Office of Inspector General issued earlier this month about conditions and practices at hundreds of immigration detention facilities. “We are concerned that the OIG report, while horrific in its findings, merely scratched the surface of systematic abuses and violations reported by whistleblowers,” GAP said in its letter.

“We are concerned that the OIG report, while horrific in its findings, merely scratched the surface of systematic abuses and violations reported by whistleblowers.”

The group is representing multiple immigration whistleblowers, including Ellen Gallagher, who went public with her concerns about ICE’s use of solitary confinement in interviews with the International Consortium of Investigative Journalists and The Intercept last month, following her yearslong effort to raise the issue before governmental oversight bodies.

The inspector general’s recent report “substantively confirmed Ms. Gallagher’s disclosures, made over a period of almost five years, [but] they only exposed the tip of the iceberg,” the watchdog group wrote to members of the Judiciary and Oversight committees in the House, as well as the Senate’s Judiciary and Homeland Security committees. “Ms. Gallagher’s disclosures documented hundreds of examples of ICE’s inappropriate use of solitary confinement, which have yet to be investigated by internal DHS watchdogs. This suggests her concerns are widespread across hundreds of facilities and need to be investigated and addressed immediately.”

Gallagher first became aware of ICE’s misuse of solitary confinement in 2014, when she worked at the DHS Office for Civil Rights and Civil Liberties. After reviewing the agency’s segregation reports, she spent the next several years trying to bring the issue before her superiors at DHS, the Inspector General’s Office, the Office of Special Counsel, and congressional oversight committees. Her review of the reports, in addition to case files for some detained immigrants, led her to believe that ICE’s widespread use of solitary confinement, particularly with regard to individuals with “special vulnerabilities” — such as those with serious mental illness — violated the agency’s policies and procedures.

Gallagher said she hopes that GAP will bring critical attention to a dire issue. “Thousands of civil immigration detainees continue to be ‘sentenced’ to solitary confinement where they are denied proper medical care and attention,” Gallagher wrote in an email to The Intercept. “I believe GAP’s advocacy will be instrumental in halting egregious violations of law and policy — and in saving lives.”

The issues she raised were consistent with the results of the investigation, conducted by ICIJ and The Intercept, which included a review of more than 8,400 records of placement in solitary confinement from 2012 to early 2017. The investigation found that ICE was using solitary confinement as a go-to tool, often to the detriment of detained immigrants with mental illness or a physical disability, or LGBTQ individuals who are especially vulnerable in immigration detention.

The Government Accountability Project letter, authored by attorney Dana Gold and national security analyst Irvin McCullough, decried the failure of oversight agencies to respond to Gallagher’s disclosures. The Office of Special Counsel “expressly abdicated its duty to investigate Ms. Gallagher’s disclosures,” the letter reads. “Rather than act upon these urgent pleas and the reams of information associated with them, the OSC instead found it acceptable to defer to the OIG’s own incomplete investigation”

Gold and McCullough called on each of the committees to hold a two-panel hearing to evaluate the June 2019 Inspector General report, ICE detention more broadly, and the executive branch’s oversight failures. “One panel should represent the internal watchdogs responsible for immigration policy” — the DHS inspector general, the civil rights and civil liberties director, and the special counsel — while the other “should allow whistleblowers and subject matter experts, who have primary knowledge of detention facility conditions, to testify to the extent of these violations and pinpoint avenues for further oversight.”

The issue of ICE’s use of solitary confinement has seen its profile rise in recent weeks, following The Intercept and the OIG’s reports. In recent days, two Democratic Party presidential contenders weighed in with calls for accountability. On Thursday, Sen. Cory Booker, D-N.J., a member of the Senate Judiciary Committee, called for the Republican-led committee to hold hearings on the subject. His letter came several days after Sen. Elizabeth Warren, D-Mass., wrote a letter to ICE leadership demanding answers to a series of questions.