(AP) — The fate of thousands of migrant families who have recently arrived at the Mexico border is being decided by a mysterious new system under President Joe Biden. U.S. authorities are releasing migrants with “acute vulnerabilities” and allowing them to pursue asylum. But it’s not clear why some are considered vulnerable and not others. Some families that apparently don’t meet the criteria are being expelled under pandemic-related powers that deny them an opportunity to seek humanitarian protection. The mystery keeps migrants guessing. Many believe the key factor is the presence of young children. The criteria to be allowed into the U.S. are a closely held secret.