Trump's "border czar" claims Mexican cartels killed a quarter-million Americans with fentanyl. Here's a fact check.

President-elect Donald Trump has said halting fentanyl trafficking across the southern border is a top priority for his administration, vowing to impose 25% tariffs on all imports from Mexico until the flow of "drugs, particularly fentanyl, and illegal immigrants" into the United States is stopped to his liking.

Trump has claimed fentanyl overdoses kill 300,000 people annually, a toll he said is "probably much more," while incoming "border czar" Tom Homan alleged in a Fox News interview this week that Mexican cartels have "killed a quarter of a million Americans with fentanyl."

Synthetic opioids such as fentanyl have become the leading cause of overdose deaths since 2016, devastating communities across the U.S. and causing a major public health challenge, according to the National Institutes of Health. Mexican cartels are the main source of finished fentanyl in the U.S., officials say.