By Justin Bachman, Bloomberg American Airlines and United Continental asked the U.S. government not to fly immigrant children separated from their families on their aircraft as President Donald Trump said he was abandoning his “zero tolerance” border-enforcement policy. In joining critics of the U.S. detention of the youngsters, the carriers highlighted a central mystery in the political and human-rights crisis: Federal officials weren’t saying how the children were being ferried from near the U.S.-Mexico border to a network of facilities in 17 states. “Based on our serious concerns about this policy and how it’s in deep conflict with our company’s values, we have contacted federal officials to inform them that they should not transport immigrant children on United aircraft who have been separated from their parents,” United Chief Executive Officer Oscar Munoz said Wednesday in an emailed statement.