Nearly 30 percent of the illegal immigrant children the U.S. is holding in its dormitories have ties to criminal gangs, the government revealed Wednesday, suggesting that the Obama-era surge of Central Americans has fed the country’s growing problem with MS-13 and other gangs. Federal officials refused even to guess at the true scope of the problem, telling the Senate Judiciary Committee that they can give only small snapshots of what they see. But they said the devastation on communities across the country is clear: killings and chaos, particularly among other immigrants — both legal and illegal. The Border Patrol identified...