Family separations at the border have ended, but at least 2,300 children and parents who were separated since April must now be reunited, and there seems to be no clear plan for how that will happen.

But there is one tool that, in theory, should make reunification more feasible: the Alien Registration Number, or “A” number, that every immigrant is issued when they're processed by ICE. Since those numbers are often issued sequentially, parents and their advocates have been using them to deduce where separated children are located and vice versa, even if they don’t know the actual number itself.

Here’s how it works: When a parent and child who have been detained at the border are processed, they’re given their alien registration numbers, commonly known as “A” numbers. These numbers identify individuals who are seeking to stay in the U.S., and remain with them throughout immigration proceedings. “A” numbers allow the government to keep track of people in the system.

When separated children are sent from ICE custody to shelters run by the Office of Refugee Resettlement, their “A” numbers are included in their case file, along with identifying information like name, age, date of birth, and country of origin, according to emails VICE News obtained from a large government agency. On top of this, the reason why the child was separated from the parent is noted, and the parent’s “A” number is often (though not always) included in the file.

This means that if a parent in detention knows his or her child’s “A” number, he or she can call the 24-hour parent hotline run by ORR and find out exactly which shelter the child was sent to. You can also simply give ORR your child’s name, but the handout given to parents with instructions for locating their child recommends giving the child’s “A” number to the operator. The problem, though, has been that many parents don’t know their child’s “A” number, and have no easy way of getting it. (It’s not clear if parents’ case files include their children’s “A” numbers, or if ICE officers will give the parents that information if it’s available.)

What VICE News has found, though, is that there's a pattern to the way “A” numbers are issued. In some cases, parents and children who are apprehended together are issued the same A number. More often, the numbers are different, but only by one number. So if a parent has an A number ending in 40, there's a good chance her child’s A number ends in 39 or 41. For a mother traveling with three kids, the numbers will typically be four in a row. So if the mom’s number ends in 39, her kids’ numbers are likely to end in 40, 41, 42, or 36, 37, 38.

The pattern is not perfect: We have found cases where a parent and child’s numbers are several digits apart. But for parents trying to track down their children, understanding the numbering system is potentially a breakthrough in a reportedly confusing and complicated process. A parent in detention may not know her child’s number, but she will know her own, and if she knows her own, she can make an educated guess about what her child’s number will be. And that will improve her chances of finding her child if she calls the ORR hotline.

Of course, locating the child is just the first step. Even if contact is made, there is no confirmed process for how that reunification might happen.