Something unexpected is happening at U.S. military bases hosting Afghan evacuees: Many hundreds of them are simply leaving before receiving U.S. resettlement services, two sources familiar with the data told Reuters.

The number of “independent departures,” which top 700 and could be higher, has not been previously reported. But the phenomenon is raising alarms among immigration advocates concerned about the risks to Afghans who give up on what is now an open-ended, complex and completely voluntary resettlement process…

But leaving early could cost other Afghan evacuees critical benefits – like expedited work permits – and create a slew of legal problems down the road, given the complexities of the U.S. immigration system.

“It’s a giant can of worms,” said one U.S. Citizenship and Immigration Services official, who spoke on condition of anonymity.