More than 15,000 illegal immigrants traveling as families were nabbed at the U.S.-Mexico border in November — a massive increase that marks the worst November on record, and the second-worst overall, according to new statistics released Thursday by Homeland Security. The number of children traveling without parents also ticked up, topping 7,000 for the month, but it’s the surge of families that’s straining the Border Patrol and testing the Obama administration’s resolve. Combined, the children and families fleeing Central America for the U.S. have reshaped the challenges of the illegal migration problem, sending the overall level of illegal immigration back to levels that haven’t been seen in years. November’s 47,214 illegal immigrants caught is 44 percent than the level in 2015, and is the worst November in years. Obama officials blame conditions in Central America, saying poverty and violence in El Salvador, Honduras and Guatemala are forcing people to make the trip north. But the Border Patrol’s chief told Congress that U.S. policy is inviting the surge because migrants, coached by the smugglers they’re paying, have learned to gain the system. The worst month for children and families is June 2014, which was the peak of the previous surge. But migration is cyclical, and the fact that this year had the worst November on record suggests fiscal year 2017, which began in October, is poised to set new records for families.