President Trump continues making the case for a wall on the U.S.-Mexico border, saying it's the only way to stop the flow of undocumented immigrants and illegal drugs. So are illegal drugs actually coming into Massachusetts via the U.S.-Mexico border? WGBH News' Morning Edition host Joe Mathieu spoke with Andrew Lelling, the U.S. Attorney for Massachusetts, in a wide-ranging interview in his office about the opioid epidemic. The transcript below has been edited for clarity.

Joe Mathieu: Are the drugs that are reaching places like Lawrence and Boston, throughout Massachusetts, coming over the border? Are these tied in?

Andrew Lelling: Yes, they're tied in. Events at the border don't directly impact what happens in Massachusetts, but I can tell you that the drugs we see on the streets of Massachusetts come from a few different places. They come from Mexico through the southwest border, some come directly from China to here, some come from the Dominican Republic to here. The origin of, say 80 percent, just to pick a number, the vast majority of these drugs is ultimately Mexico or China. It's just a matter of what route they take to find their way to the Commonwealth.

Mathieu: So when we talk about the Dominican Republic or other areas outside of the border, the source is the same, the point of origin is the same, they're just finding different ways to get it here?

Lelling: Yes, that's exactly right.

Mathieu: Is that the same now as it was a year ago and we talked about it or you constantly kind of rerouting your efforts to follow the trail of drugs?

Lelling: No, that's basically the same as a year ago. Again, the primary sources of the illegal drugs — and when and I say illegal drugs I mean heroin and especially fentanyl — is China, which has a poorly regulated pharmaceutical industry, both on the legal side in China, and there are obviously unregulated illegal outfits in China that generate precursor chemicals for making fentanyl, and which just make fentanyl. And they export it to Mexico or other points in North America, and they make their way into the U.S. and a lot of that fentanyl, by one route or another, makes its way to Massachusetts. And so when we do drug investigations here in Massachusetts, we are often trying to start at the street level and work our way back up the supply chain to find the highest level supplier we can and prosecute those bulk traffickers to try and keep the drugs off the street.

Mathieu: How busy are you, for instance, at Logan Airport in that effort?

Lelling: I think activity at Logan remains relatively constant. There are federal agents posted at Logan Airport. You have the Department of Homeland Security there. There are DEA agents there. The state police are active there. And so there is a steady rate of interdiction of drugs coming into Logan Airport. I don't think that's the biggest source of drugs that we see in Massachusetts. Most of the drugs we see here are coming in by car or by truck.

Mathieu: We're talking with U.S. Attorney Andrew Leslie. You wrote an op-ed talking about the opioid epidemic as the biggest public health crisis, you said, in the U.S. since the Spanish Flu in 1918. That is quite a line.

Lelling: Opioids killed 72,000 people in the United States last year, which is an astounding figure. Some high proportion of that figure was fentanyl-related deaths. Fentanyl has taken over the opioid crisis. It used to be, for example, that we worried more about heroin itself, or we worried more about prescription painkillers. Now we worry most about fentanyl. The proportion of opioid deaths that are fentanyl related deaths continues to climb steeply every year.

Mathieu: Your op-ed of course was about the idea of safe injection sites, and you've taken a hard line on that, after our mayor and the mayor of Cambridge traveled to Canada to inspect one that was operating there. You wrote, 'Any effort to open injection sites will be met with federal enforcement.' That's a pretty hard line.

Lelling: Well, it was important to me, and I thought it was in the public interest, to just be clear about this. So everyone goes in with their eyes open. There is a federal statute that, as I read it, that makes supervised injection sites simply illegal. And, in light of the fact that opioids are killing thousands upon thousands of people a year, I see no reason to support an effort to essentially normalize opioid use, which is what I think supervised injection sites do. The record is mixed for these. Some studies say they work, some studies say they don't. Overall, the studies seem to imply some benefit from them, though it appears difficult to measure. But that's not really where I think the issue is for me. The issue for me is that these sites are currently illegal under federal law. The people who support them should be going to Congress in an effort to change the law. Until that happens my office will enforce it.

Mathieu: You spoke in a similar fashion about marijuana enforcement when it comes to the state law versus the federal law. And we're reading recently reports that the black market has really not been impacted, in fact may be growing even as the recreational marijuana business takes hold in our state here. To what extent are you involved on the federal level in stopping the trafficking of marijuana or grow sites that might be breaking the law?

Lelling: We keep doing marijuana cases. We've never stopped doing marijuana cases. The cases that we tend to focus on are large-scale, bulk trafficking of marijuana and maybe related to money laundering. But we do this on a case-by-case basis. I can't immunize the whole state of Massachusetts from prosecution under federal laws that make it illegal to traffic in marijuana. We continue to do those cases. The difference is that I have to decide where to put my resources. I have 15 or so drug prosecutors. Opioids are killing thousands of people a year. Marijuana is not killing thousands of people a year. And so the resources I've got are primarily focused on the opioid trade, and not the marijuana trade. But as these people would have seen in the press over the last few weeks, we do continue to do certain marijuana cases.

Mathieu: I think you've made a line in your op-ed that said, 'Heroin is not the same thing as a pot brownie.' What did you mean by that?

Lelling: What I meant by that is that I think that some people who support the push, first to legalize marijuana, and then second to pursue the opening of a supervised injection sites, are just a little too cavalier about the impact of these illegal drugs. They're illegal for a reason. There are significant negative health effects associated with marijuana. And I think some of those health effects were minimized in the public debate over marijuana legalization. But that's in the past. The state chose to do that. Now that we see a debate ramping up about supervised injection sites, one of the reasons why I wrote the op-ed is I don't want to see that same mistake again. There are serious concerns to think about when it comes to supervised injection sites, especially because they will, in some ways, normalize intravenous drug use of opioids. And I think that is a mistake. And I want people to think about the potential downsides before the snowball gets bigger and bigger, leading towards opening one of these sites in Massachusetts.