The sides in an ongoing lawsuit over the separation of migrant families at the U.S.-Mexico border are due in court Thursday afternoon.

An independent federal investigator has said there may be thousands of migrant children who were separated from their parents prior to last year's court-ordered reunifications.

Federal officials have said identifying them all means sifting through tens of thousands of cases and could take years. But the American Civil Liberties Union said that’s too long.

The judge overseeing the case asked both sides to work on a deal, says Mark Greenberg, senior fellow at the Migration Policy Institute in Washington, D.C.

“And if they haven’t been able to reach agreement, then the court may need to make an order and decide this issue,” he said.

Cmdr. Jonathan White, the government official who led last year’s court-ordered reunifications, is expected to be at Thursday’s hearing.