Thursday is the court-ordered deadline for the Trump administration to reunite more than 1,500 children separated from their parents after crossing into the United States illegally in recent months.
Of the 2,551 children identified by the federal government as "potentially" covered by the court order, 1,012 had been reunited with their parents by Monday night.
The government anticipated it will have reunited some 1,600 families found to be "eligible" for reunification by the Thursday deadline.
Homeland Security Secretary Kirstjen Nielsen reportedly told members of the Congressional Hispanic Caucus on Wednesday that officials are "on track" to meet the deadline. But the meeting angered some lawmakers, who told Nielsen the deadline was unrealistic.
Some parents were rejected for reunification because of a criminal record the U.S. government said disqualified them to reclaim custody; others were deported or returned voluntarily to their home country, complicating reunification.
Federal officials said 463 parents already may have been deported without their children.
Illinois Democrat U.S. Rep. Luis Gutierrez, a member of the Hispanic Caucus, accused Nielsen of "committing crimes against humanity" and called her "an accomplice of Donald Trump's racist regime."
Under the Trump administration's so-called "zero-tolerance" policy, adults who crossed into the United States illegally in much of April and May were detained; any children traveling with them were taken to separate detention centers. Visitors said the children were held behind fences in wretched conditions and were given little to occupy their time.
Trump signed an executive order rescinding the "zero-tolerance" policy after a nationwide outcry, including many from his own Republican Party.
Federal judges gave the administration deadlines to reunite families and turned down the government's request for more time.
The Department of Health and Human Services said it takes time to determine whether the children can be safely sent back with their families, noting that some of the illegal immigrants may be child abusers or wanted for violent crimes.