Earlier this month, a federal judge ruled that the Trump administration must seek the return of a migrant who was mistakenly sent off to prison in El Salvador.

Although the White House appealed her ruling to the Supreme Court, it upheld the order in a unanimous decision on Thursday.

Administration admits that Kilmar Abrego Garcia was wrongly removed

According to the New York Post, the Trump administration pointed out in its Supreme Court petition submitted last week that Kilmar Abrego Garcia is currently being held in El Salvador's Center for Terrorism Confinement (CECOT).

Solicitor General John Sauer thus maintained that the White House "cannot guarantee success in sensitive international negotiations" concerning Garcia’s release.

The solicitor general did concede that Garcia’s deportation to El Salvador had been the product of an "administrative error."

Nevertheless, Sauer maintained that this fact did "not license district courts to seize control over foreign relations, treat the Executive Branch as a subordinate diplomat, and demand that the United States let a member of a foreign terrorist organization into America tonight."

Case remanded to lower court for further instructions

However, all seven members of America's highest judicial body were unpersuaded by Sauer's ruling and remanded the case to Maryland District Court Judge Paula Xinis for further instructions on how the administration can bring about Garcia's return.

They pointed out how Garcia had been "subject to a withholding order forbidding his removal to El Salvador," thus making his deportation unlawful.

"The [lower court] order properly requires the Government to ‘facilitate’ Abrego Garcia’s release from custody in El Salvador and to ensure that his case is handled as it would have been had he not been improperly sent to El Salvador," the opinion read.

"The intended scope of the term ‘effectuate’ in the District Court’s order is, however, unclear, and may exceed the District Court’s authority," it continued.

Judge grills deputy attorney general in "tense" hearing

"The District Court should clarify its directive, with due regard for the deference owed to the Executive Branch in the conduct of foreign affairs," the justices explained.

"For its part, the Government should be prepared to share what it can concerning the steps it has taken and the prospect of further steps," they went on to add.

The Post noted how on Friday, Judge Xinis grilled Deputy Assistant Attorney General Drew Ensign in what the newspaper described as a "tense" hearing.

"I'm not asking for state secrets," Xinis was quoted as saying. "All I know is that he’s not here. The government was prohibited from sending him to El Salvador, and now I’m asking a very simple question: Where is he?"