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(CNN) —  

In an effort to justify new tariffs on Mexico, President Donald Trump decried what he sees as the country’s failure to stop the import of illicit drugs into the US via the southern border.

“90% of the Drugs coming into the United States come through Mexico & our Southern Border,” Trump tweeted. “This has gone on for many years & nothing has been done about it.”

It’s important to note upfront that Trump’s claim is not about the origin of where these drugs are manufactured, but rather where they enter the US. Much of the data is derived from drug seizure statistics, which provide a less-complete picture of where these drugs are coming from.

Facts First: While a majority of illicit drugs enter the US through the southern border when considering measurements like the purity of certain drugs, it’s unlikely that Trump’s 90% figure is accurate.

Due to the difference in the purity of particular drugs, it’s difficult to measure what percentage of total drugs are coming from which country since the primary measure (gross weight) provides an incomplete picture. For instance, in the US Department of Justice Drug Enforcement Administration’s 2018 National Drug Threat Assessment the agency notes that it is impossible to determine what country supplies the most fentanyl.

The DEA says that due to rates of purity, “It is currently not possible to determine which source, Mexico or China, is the greater direct threat as a supplier of fentanyl to the United States.” While Mexico imports a larger total weight of fentanyl than the US seizes at the border, the fentanyl coming directly into the US from China has an “exceptionally high purity,” that can then be diluted to create more drugs.

The DEA also notes that Mexican traffickers get the majority of their supply for chemicals necessary to manufacture fentanyl from China.

In a statement to the Senate’s caucus on international narcotics control in October 2018, the US Immigration and Customs Enforcement noted that “US law enforcement has identified China as the primary source of illicit fentanyl and fentanyl analogues entering our country.”

When it comes to heroin, Trump’s 90 percent figure is more accurate. In 2017, William Brownfield, the assistant secretary of International Narcotics and Law Enforcement Affairs, told reporters that he estimated “between 90 and 94 percent of all heroin consumed in the United States comes from Mexico.”

The DEA reported that “Heroin from Mexico accounted for 86 percent of the heroin by weight analyzed” in 2016 and that the southwest border “remains the primary entry point for heroin into the United States.”

As for cocaine, the majority of the drug that winds up in the US comes from Colombia and enters through the southern border with Mexico. According to the DEA, 93 percent of cocaine samples tested were of Colombian origin. The Southwest border is the “principal entry point for the majority of the cocaine entering the United States,” according to the DEA. The majority of seizures of cocaine coming into the US take place at ports of entry or Border Patrol checkpoints.

In its analysis of methamphetamine, the DEA also found that the southwest border “remains the main entry point for the majority of methamphetamine entering the United States” accounting for 97% of meth seizures by the Customs and Border Patrol.

Similarly, the report states that “Mexico remains the most significant foreign source for marijuana available in the United States,” however the rate of foreign marijuana continues to decrease as domestic production continues to grow as more states legalize the drug.