CHICAGO (CBS) — Thursday is the deadline for the U.S. government to reunite migrant families separated at the border, but hundreds of parents likely will not be back with their children by the deadline.

The federal judge who ordered that separated families be reunited commended the government’s progress so far, with more than 1,000 families reunited.

However, more than 900 parents won’t be reunited with their children by the deadline.

More than 400 parents are no longer in the U.S., likely deported without their children, according to lawyers familiar with court records.

In other instances, the children’s families can’t be located.

In about 200 of those cases, the parents have serious criminal records, or declined to be reunited. Immigration advocates said some parents are feeling pressured to sign away their rights to be reunited with their children.

Attorneys claim those parents don’t understand what they’re agreeing to; whether the result of a language barrier, or documents that are too complex.

U.S. Sen. Dick Durbin and U.S. Rep. Luis Gutierrez repeatedly have slammed the situation that created this mess: the Trump administration’s zero tolerance policy regarding immigrants caught crossing the border illegally, even those seeking asylum.

“The reality is today, were it not for a federal judge in San Diego, who demanded this reunification, these children would be cast to the wind for weeks and months further,” Durbin said. “Yesterday it was reported there were 37 they could not identify, in terms of the identity of the child or the parent. That is an incredible number. Thirty-seven lost children in the United States of America because of our government policies. I’m fearful the number’s even larger than that.”

Gutierrez vowed to hold Homeland Security Secretary Kirstjen Nielsen’s “feet to the fire” on the family separation policy.

“If she thought she was uncomfortable trying to eat in a restaurant in Washington D.C., that will be nothing compared to the heat she will face when we speak today,” he said Wednesday, referring to a June incident when protesters heckled Nielsen at a Mexican restaurant in Washington. “We are going to speak clearly, and unequivocally about the needs of these children, about the terror that they are confronting, because that’s our responsibility; if we do not speak for them clearly and without reservation, then who will?”

About two weeks ago, the Department of Homeland Security reported it completed reunification of families with eligible children under age 5. The deadline to reunite those children with their parents was July 12, but the Trump administration missed that deadline for many eligible families.