(CNN) The Trump administration could face a major shakeup soon in the lawsuit over immigrant families separated at the border.

A judge presiding over the ACLU's case is weighing a key question:

Should the government be required to comb through a massive trove of records and identify potentially thousands of additional families who were separated?

ACLU attorneys are pushing for this, but government attorneys vehemently oppose it.

US District Judge Dana Sabraw on Thursday vowed to rule as quickly as possible. Based on his comments in court, he isn't inclined to side with the government.

If that's the case, it's a big deal. It could mean the government will be ordered to undertake tracking efforts like we saw over the summer , but on an even larger scale.

Here's a look at what we learned in the latest court hearing, what's going on, and why it matters:

The judge repeatedly emphasized the importance of "an accounting."

Thanks to a report from the Health and Human Services Inspector General, we recently learned that potentially thousands more parents and children were separated by the US government than officials had indicated.

Those families weren't covered by Sabraw's June ruling in the case , which spurred a massive effort to reunite thousands of immigrant families the US government had separated. That's because this other group of separated children had been released from government custody already.

So we don't know who those families are or what's happened to them.

That didn't seem to sit well with Sabraw as he pressed government attorneys on Thursday.

"Isn't it important to have an accounting, simply an accounting of what happened to whom, how many are involved and where are they?" he asked.

The government really, really, really doesn't want this to happen.

Justice Department attorney Scott Stewart deployed a range of arguments to convince the judge that expanding the case to include families separated before June 26, 2018 isn't a good idea.

He repeatedly emphasized what a significant undertaking it would be for the government to compile a new list of separated families.

Requiring that, he said, would "blow the case into some other galaxy of a task." He called the notion a "seismic shift" and a "dramatic change."

The ACLU says it's willing to do whatever it takes to track down separated families.

ACLU attorney Lee Gelernt said the organization is ready to set up another steering committee to track down separated families.

"We are prepared, no matter how big the burden is, to take that on," he said, noting that the consequences of not doing so could be dire.

"We're talking about little children potentially being permanently orphaned," he said.

Even if the judge orders the government to identify these families, they might not be reunited.

After Sabraw's order over the summer, officials identified separated families and began a painstaking process to reunify them.

But there's no guarantee we would see the process play out the same way this time around.

Sabraw noted on Thursday that reunifying families is only one potential remedy.

"The alleged wrong is the government conduct of separating families as a matter of policy," he said.

The appropriate remedy for these other families, Sabraw said, could be determined later.

"If this motion is granted, step one, which is a very significant step, would be the accounting," he said. "What are the numbers? Who are they? Where are they?"

Pinpointing what happened is important, the judge said.

"In an allegation of wrong on this scale," he said, "one of the most fundamental obligations of law is to determine the scope of the wrong."