Fentanyl, the opioid that is up to 50 times as potent as heroin, is presenting law enforcement with complex challenges, according to Richard Baum, Acting Director of the White House Office of National Drug Control Policy. In a letter to U.S. House legislators, Baum said fentanyl is coming into the country from an array of sources, The Wall Street Journal reports. He called fentanyl “an urgent public health threat.” Just 2 milligrams of powdered fentanyl can be deadly. Authorities seized at least 668 kilograms of fentanyl last year—enough to kill every American. Seizures of fentanyl in liquid and pill form have also been increasing.