In their ongoing push for $5.7 billion to expand the border wall, Trump administration officials have repeatedly pointed to the flow of drugs across the southern border as proof that such a wall is needed.

President Donald Trump has used that line. White House press secretary Sarah Sanders has, too.

But an analysis of data from the southern border indicates that the vast majority of narcotics enters through U.S. ports of entry, not the wide swaths of border in between where additional barriers could be erected.

According to U.S. Customs and Border Protection statistics, 90 percent of heroin seized along the border, 88 percent of cocaine, 87 percent of methamphetamine, and 80 percent of fentanyl in the first 11 months of the 2018 fiscal year was caught trying to be smuggled in at legal crossing points.

While those numbers deal only with drugs that are caught, border experts say the data accurately reflect the way drug cartels successfully smuggle narcotics into the country.

Vice President Mike Pence:Democrats refuse to compromise on border wall funding to end the shutdown

Gil Kerlikowske, who headed CBP and the Office of National Drug Control Policy under President Barack Obama, said intelligence received from arrested smugglers and law enforcement partners in Mexico indicate that cartels clearly prefer moving high-profit narcotics through the busy ports of entry because their chances of success are better there.

He used the example of the San Ysidro Port of Entry in southern California, the busiest port with 100,000 people crossing through each day. Port officials recently completed a multi-year, $750 million upgrade to add more Customs officers and inspection technology, but Kerlikowske said the sheer volume of traffic means smugglers' odds are still better going through there than other parts of the border.

"Regardless of the number of drug dogs and technology and intelligence, the potential of smuggling the drugs in through a port of entry is far greater. Your ability to be captured coming across between a port of entry is much greater," said Kerlikowske, now a professor of practice in criminology and criminal justice at Northeastern University. "It's very clear that (drugs) come through the ports."

The flow of illegal drugs into the U.S. has become a central part of the Trump administration's argument that more barriers are needed along the 2,000-mile southern border. Trump's insistence on $5.7 billion for funding led to the ongoing, partial government shutdown, already the longest in U.S. history at 25 days.

Democrats have rejected Trump's request, instead preferring to invest more heavily in ports of entry. The Democratic-led House of Representatives passed a spending bill Jan. 3 that includes $8 million to hire 328 new Customs officers and $225 million to purchase equipment used to screen trucks and vehicles for contraband.

While Trump officials usually mention that 90 percent of the narcotics that enter the U.S. comes across the southern border, they have usually left out the way in which those drugs cross. Vice President Mike Pence was the first to acknowledge in an op-ed column published in USA TODAY on Tuesday that those drugs "primarily" enter the country through ports of entry.

Trump has argued the exact opposite, saying before a trip to McAllen, Texas, last week that most drugs enter between "portals" along the border, not through those "portals."

That is wrong.

Perhaps the best authority on how drugs are smuggled into the U.S. is Joaquin "El Chapo" Guzman, the Mexican drug lord arrested by Mexican officials in 2016 and later extradited to the U.S.

During his ongoing trial in New York, several of his cartel members have testified that they mostly pushed drugs through U.S. ports of entry, stashing bricks of narcotics in cars, trucks and trains, according to CNN. None of El Chapo's associates has testified that they moved drugs through the open border regions in between those ports.

Recent headlines also highlight the fact that drug cartels have many ways of getting around – or over, or under – any proposed border wall.

In the past month, Mexican authorities have discovered three tunnels that were used for drug smuggling that crossed under the border wall into Arizona. That has long been a common smuggling route used by smugglers.

In December, Border Patrol agents arrested two men after an ultralight aircraft flew over the border wall in California and dropped $1.4 million worth of methamphetamine and a getaway bicycle. And last year, a man was sentenced to 12 years in prison after he was caught picking up a package with 13 pounds of methamphetamine that had been dropped by a drone that flew over the border wall near San Diego. Border Patrol officials say such airborne smuggling attempts are only expected to increase as drone technology improves.

Smugglers also rely on the U.S. Postal Service to smuggle their highest-profit drugs into the country. A September report from the Postal Service's Inspector General found that drug shipments have been on a steady rise in recent years, with over 40,000 pounds of drugs seized in the mail in 2017.

The report concedes that postal inspectors identify only a fraction of the drugs entering through the U.S. mail. It found 104 drug trafficking websites on the dark web that identified their shipment methods, and 92 percent indicated they used the U.S. Postal Service.

The ability of smugglers to take advantage of the U.S. Postal Service has become so acute that it prompted a special agent in November to say: "Postal employees are paid to deliver mail, not drugs."