According to [Fentanyl seizures at US-Mexico border up 750 percent][1]:


>The amount of fentanyl seized by border agents has skyrocketed in recent years, from two pounds in fiscal year 2013 to approximately 1,485 pounds from Oct. 1, 2016, to Sept. 30, 2017  

Matthew Barden of the Drug Enforcement Administration is [cited][2] as saying: 

>300 pounds of fentanyl is lethal enough to kill about 100 million people


So enough fentanyl was seized by border agents in FY2017 to kill every American.  As long as 50% makes it through (and 2018 is at least as much as FY2017), the claim is defensible.  



  [1]: https://www.washingtonexaminer.com/news/fentanyl-seizures-at-us-mexico-border-up-750-percent
  [2]: https://www.omaha.com/news/crime/pounds-of-fentanyl-among-drug-bust-records-this-year-for/article_37809613-706c-54a3-80c6-7c5a3c2b18a3.html