(CNN) Machines used to make counterfeit pills are reaching the US border in record numbers.

US Customs and Border Protection is seizing pill presses at a rate 19 times higher than in 2011. That's the year the synthetic drug fentanyl exploded in the US drug market, according to the American Association of Poison Control Centers.

While the Justice Department's Drug Enforcement Administration deals with a national problem of synthetic fentanyl overdoses, these machines are playing a critical role in getting deadly counterfeit drugs onto American streets.

"To the naked eye, you can't tell the difference," said John Martin, special agent in charge of the DEA's San Francisco division. "If you have counterfeit pills, you can't make them without pill presses."

Counterfeit pills marketed as oxycodone or Xanax but made with fentanyl can be deadly. Fentanyl is 25 to 50 times stronger than heroin and up to 100 times more potent than morphine.

The painkiller was once found only in pharmacies but is now manufactured in China in synthetic form and widely available on the American black market.

Pill presses -- which can easily be bought online -- allow someone to take powder and press it into a pill that looks legitimate. "People have died from ingesting what they think is a legitimate painkiller, (but really) it's a counterfeit pill that contains fentanyl," Martin said.

Across the country, authorities have seen this play out.

The death of pop icon Prince may turn out to be one of the most famous cases of counterfeit pills. Police reportedly found mislabeled pills laced with fentanyl in his home, and the drug was found in his system.

And with the machines easily available, the demand high and the setup easier than ever, it means the potential profits are huge.

Forty-three percent of America's goods from outside the country arrive at the Port of Long Beach, California, according to US Customs and Border Protection. Authorities hold questionable pill press machines while they investigate whether they are coming into the country legally.

Most come from China. That's where much of the illegal fentanyl is manufactured, as well. Clandestine Chinese labs manufacture a synthetic version of fentanyl that is easily bought on the dark Web. Mexican drug cartels later began to buy it and resell it across the border.

Pill press machines themselves are not illegal, as long as they are properly registered with the DEA before they are shipped. But many of them are not.

These devices vary in size from giant industrial electric ones, bigger than refrigerators, to plastic tabletop machines. Some can generate 170,000 pills per minute. CNN got exclusive access to the warehouse where they are kept.

"With all the overdoses we're seeing, the increase in overdoses in the last couple years, I think these types of interceptions are extremely important," said Cheryl Davies, assistant port director at Los Angeles International Airport. "They have a lot of impact on our communities."

Finding the pill presses is difficult. Every 7.8 seconds, a new container is processed at the port. By checking manifests for key words and electronically scanning suspicious containers, customs agents have been able to find and seize dozens of illegally imported presses. Since 2013, there have been more than 80 seized.

When they find one that is not properly registered, they alert law enforcement.

In 2015, Customs alerted the DEA about one such pill press that arrived from China. The DEA got a warrant to place a GPS tracker inside the shipment and traced it to Gary Resnik, a 32-year-old man. When they searched Resnik's home, agents found six pill presses and nearly 30 pounds of acetyl-fentanyl. The DEA alleges that Resnik and three other men were importing the synthetic opiate, making their own pills and selling them in bulk. Authorities later charged Resnik and others with federal narcotics and money laundering in the Los Angeles area.

Martin explained that pill presses have made it easier for drug dealers to make millions of dollars out of their homes, without being affiliated with any kind of big drug cartel.

It's cheap and easy to order wholesale fentanyl powder and a pill press online. For $5,000 to $6,000, Martin said, someone could potentially make $10 million worth of fake pills.

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"A kilogram of fentanyl wholesale is about $3,500 to $5,000. The pill press, let's say you buy it for about $1,000 and the die for $100, that's not a huge investment. You get the binding material ... on the dark net, and you can start making these pills," he said. "So there's huge profit to be made on these counterfeit pills."

The acting DEA administrator, Chuck Rosenberg, went to China in January to meet with officials about keeping synthetic fentanyl and pill presses from being shipped illegally over to the US.