The Supreme Court on Monday agreed to hear a Justice Department appeal of a 2015 lower-court decision requiring bail hearings for immigrants who have been in detention for at least six months awaiting deportation proceedings.

However, the American Civil Liberties Union—which won a lower-court ruling requiring bail hearings after six months—said recently disclosed hearing records show a​2003 high-court precedent the Justice Department cited to bolster its case was partly based on government-supplied information that understated the length of immigration detentions.

It isn’t clear whether a difference in the time frame would have affected the outcome of the 2003 case. But critics of the government’s immigration policies say that prehearing detention with no chance for bail becomes less reasonable the longer it lasts.

The 2003 case, Demore v. Kim, upheld by a 5-4 vote the government’s practice of holding without bail immigrants—even those who are permanent U.S. residents with “green cards”—who became eligible for deportation because they committed a crime.

The majority opinion in that case stressed the “very limited” length of no-bail detentions at issue, relying on figures showing the average detention in 2001 was 47 days, while the 15% of immigrants who appeal a deportation order were in detention for about 4½ months. The figures were provided by the Executive Office for Immigration Review, which conducts the hearings.

The ACLU, which worked on the 2003 case, said the actual average detention time in 2001 was 2½ half weeks longer. “The real number is 65 days,” said Michael Tan, an ACLU attorney. The group learned of issues with statistics in the earlier case through a Freedom of Information Act request filed during the current litigation.

Mr. Tan said the government reached the lower number by factoring in categories of aliens that an immigration judge was required to deport—cases that are resolved quickly because there are no issues for the hearing to resolve. Mr. Tan also said the government counted as completed cases that weren’t over but only transferred—with the immigrant still in detention—to another immigration court.

Justice Department spokesman Patrick Rodenbush said officials were re-examining the numbers provided in the Kim case, but after an initial review, “we feel our information to the court was appropriate.”

The court will hear the case on bail hearings in its next term, which begins in October.

The Kim case marks the second time in recent years that a records disclosure suggested the Justice Department provided incorrect information to the Supreme Court regarding immigration practices.

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