In a warehouse several miles south of Los Angeles International Airport, a federal officer sits beside an X-ray machine, the belt moving at least three times faster than the typical airport securty one. Headphones covering her ears, she watches a small monitor as packages — often three or four across — dart across the screen.

The officer, who cannot be identified, searches mail for contraband that cannot enter the United States — items such as counterfeit clothing, illegal steroids, certain types of toys, Cuban-made cigars and fresh produce. The packages she inspects have been shipped via the U.S. Postal Service, and under federal law, officers can open anything they find suspicious.

At airports, ports and border crossings, federal officials spend a lot of time intercepting illegal drugs intended for street sales, like cocaine and marijuana, often coming from Central or South America. But at this facility, such contraband is not necessarily the biggest problem, officials say.

Of greater concern, they say, are counterfeit pharmaceuticals mailed from Asia, especially China. In many cases, the fake drugs, meant for everything from erectile dysfunction to cancer to high cholesterol, look and feel like the real thing. Often, the packaging is identical to what Americans find at pharmacies.

Customs has 38,000 square feet in the Los Angeles-area facility, one of seven international mail inspection sites nationwide. At the site, open seven days a week, 17 hours a day, officials say they process 1 to 1½ million pieces of mail each month.

Only about 5 percent of the mail is problematic, authorities say. Many of those troublesome packages carry fake drugs, which generally have three main problems: Some pills have too much active ingredient, others have too little, and some have none at all. They’re generally sold on the Internet.

“What patients don’t realize is that if they go online and they purchase from a rogue website — one that’s not legitimate — then they may be playing Russian roulette with their lives,” said Matthew Bassiur, vice president or global security for Pfizer, which makes Viagra, among other popular drugs. “It’s really that simple.”

Chris Crawford, supervisor at the U.S. Customs and Border Protection mail inspection facility near LAX, summed up fake Viagra more succinctly. “It might get you up,” he said. “But it could also kill you, you know?”

Fake drugs are such a concern for large multinational drug corporations like Eli Lilly and Pfizer that they devote teams to anti-counterfeiting measures. The teams work with federal agents and government prosecutors to help build cases against prolific counterfeiters. They also help train federal officers looking for counterfeit drugs.

Not that it’s an easy process.

“The counterfeiters are getting really good at making these look just about identical to the real products,” Bassiur said. “We have to bring it back to our laboratories to test it and find out whether it’s authentic product.” Inside fake Viagra, Bassiur said technicians have found rat poisons, heavy metals and boric acid.

Counterfeiters sending packages take considerable steps to hide their products, officials say. Sometimes, for example, they send the pills separately from the packaging, relying on someone inside the United States to put it all together. Other times, they route packages through other continents — perhaps Europe — in an attempt to elude customs officers, who might have been expecting the loot to come from China or Thailand.

Marvin D. Shepherd, a professor at the University of Texas at Austin School of Pharmacy, said counterfeiters try to copy any drug on which they believe they can make money. Erectile dysfunction drugs are particularly common for two reasons — they’re expensive and men can be ashamed to ask their doctors to prescribe them. But even expensive cancer drugs can be faked, he said.

People think — or hope — that they will work.

“They are going to counterfeit anything if they think they can sell it and make money,” Shepherd said. “That’s the way it is. They are greedy people. They don’t care about your life. All they do is care about your credit card number and getting the product sold.”

Bassiur, of Pfizer, said at least 60 of his company’s drugs have been found faked in more than 100 countries. He said Lipitor, which helps lower cholesterol, is another commonly counterfeited drug, as is Celebrex, an anti-inflammatory, Norvasc for high blood pressure, and Xanax, for anxiety. He said the number of Pfizer drugs being counterfeited has risen 300 percent since 2009.

Generally, officials say, federal agents do not arrest consumers who buy pills over the Internet, even if those customers should have known they were breaking the law. That’s not to say consumers are blameless. “You can plead ignorance,” said Crawford, of the U.S. Customs. “But you really ought to know if you are paying 10 cents on the dollar that the drugs are probably not real.”

Officials do investigate counterfeiters abroad, as well anyone who tries to smuggle in a large quality of pills. Occasionally there are prosecutions: In August, a South Korean man who attempted to carry 40,000 fake erectile dysfunction pills at Los Angeles International Airport was sentenced by a federal judge in Los Angeles to 2½ and a half years in prison.

But while a customs official at the mail facility said inspectors catch the vast majority of illicit drugs, Lawrence O. Gostin, a professor at Georgetown University Law Center, and an authority on illegal drugs, said federal inspectors actually intercept only a small fraction of illicit drugs. He said there are simply too many packages to inspect.

“We need better law enforcement to detect and prosecute all those who make, package, market or transport fake drugs,” Gostin said in an email. “Currently law enforcement is poor. The penalties are usually very low. Police also often give this area a low priority. Finally, it is a transnational problem so we need a treaty or some other international agreement to strengthen law enforcement and customs controls.”

Bill Reid, senior director of global anti-counterfeiting operations at Eli Lilly, which makes Prozac for depression and Cialis for erectile dysfunction, among many other drugs, said one of the keys is to educate consumers about the perils of fake pharmaceuticals. He acknowledged it is difficult — though not impossible — for corporations and law enforcement to go after suppliers of fake drugs.

“It’s a supply chain that is very difficult to fight,” Reid said. “There is no silver bullet in the effort to find counterfeiters.”