But Mayorkas described the claim that the bulk of fentanyl is carried into the US by migrants crossing covertly from Mexico as "unequivocally false".
"The vast, vast majority is thought to be smuggled through the ports of entry and tractor-trailer trucks and passenger vehicles," he said.
CBP reports show that last year, 84% of the fentanyl seized was found at an official border crossing.
Gil Kerlikowske, a former head of the CBP and White House drug tsar during the Obama administration, said migrants crossing the border to seek asylum have little interest in carrying fentanyl because they want to be apprehended.
"They wave down the border patrol, they want to go into custody. They want to file an asylum claim. They're not going to be carrying drugs," he said...
