Recently read that Fentanyl isn't being smuggled into the U.S. by migrants, but overwhelmingly by people with U.S. Citizenship. Who are the people smuggling fentanyl into the U.S?

Here's the article that says US citizens are the ones smuggling fentanyl: https://newrepublic.com/article/178684/fentanyl-misinformation-migrants-border-media

Is it just a part of the American drug trade now, as much as coke, heroin etc? Are large scale drug dealers sending US citizens back and forth as they're more likely to pass border checks? Who are the people that are bringing fentanyl in, and who is employing them?