The claims appear to contradict what U.S. officials have been saying.
Over 200 migrants who were sent to El Salvador's CECOT mega-prison under accusations that they were members of a violent criminal gang are the responsibility of the United States, the government of El Salvador told a United Nations working group.
El Salvador's claims, outlined in a report submitted Monday in court, appear to contradict what President Donald Trump and administration officials have been saying for months: that they are unable to return any of the migrants sent to CECOT because they're under El Salvador's authority.
"In this context, the jurisdiction and legal responsibility for these persons lie exclusively with the competent foreign authorities, by virtue of international agreements signed and in accordance with the principles of sovereignty and international cooperation in criminal matters," El Salvador officials said in their report to the U.N.
El Salvador's comments were made in a report submitted in a court filing by the United Nations Office of The High Commissioner for Human Rights Working Group, which was investigating the "disappearances" of four Venezuelan men who were thought to have been sent to the country in March.
To date, the Trump administration has not released a list of those sent to El Salvador.
"The Salvadoran State emphatically states that its authorities have not arrested, detained, or transferred the persons referred to in the communications of the Working Group," the El Salvador officials said in the report. "The actions of the State of El Salvador have been limited to the implementation of a bilateral cooperation mechanism with another State, through which it has facilitated the use of the Salvadoran prison infrastructure for the custody of persons detained within the scope of the justice system and law enforcement of that other State."
The Trump administration invoked the Alien Enemies Act -- an 18th century wartime authority used to remove noncitizens with little-to-no due process -- to deport two planeloads of alleged migrant gang members to El Salvador by arguing that the Venezuelan gang Tren de Aragua is a "hybrid criminal state" that is invading the United States.
The migrants were sent to CECOT as part of a $6 million deal the Trump administration made with Salvadoran President Nayib Bukele for El Salvador to house migrant detainees as part of Trump's immigration crackdown.
Attorneys for the ACLU who are representing the original case challenging the AEA removals -- filed in March as the men were en route to CECOT -- said they became aware of the document last week through one of the families of the men.
The attorneys accuse the government of withholding the information despite previous requests the ACLU made for documents related to the administration's arrangement with El Salvador.
"The government of El Salvador has confirmed what everyone already knew: the United States controls the fate of the Venezuelans sent to CECOT without due process," Lee Gelernt, an attorney for the ACLU told ABC News in response to the U.N. report. "Unbelievably, the U.S. government didn't provide this information to us or the court."
ABC News has reached out to the Department of Homeland Security for comment.