TUCSON — Mexican authorities uncovered another tunnel in Nogales, Sonora — across the border from Nogales, Arizona — which they suspect was used to smuggle drugs and people across the U.S.-Mexico border.

It's the third time they have made such a discovery in less than a month, and comes amid President Donald Trump's demand for funding to build more physical barriers along the U.S.-Mexico border.

Mexican Federal Police on Wednesday posted a short video of the tunnel on Twitter. Police said the tunnel measured about 32 feet in length, but offered few other details, such as the location or how they found it.

The video shows two police officers opening up a manhole underneath the extensive drainage channels that lie below the downtown areas in the twin cities of Nogales. The channels are used to ease the flow of runoff water during storms and typically flow north from Sonora into Arizona because of the terrain. But they are also commonly used in smuggling attempts.

As the video progresses, it shows the officers inside the tunnel, following it to the point where it originates at an unknown location. The officer recording is heard asking, "Is that the exit?" A second officer responds "yes" as he pushes open the entrance to the tunnel overhead.

#Nogales #Sonora. Como parte de nuestro servicio para brindar seguridad en la zona fronteriza, policías federales detectaron un túnel de casi 10 metros de longitud, presumiblemente usado para cometer ilícitos. Tu reporte es seguro y confidencial llama 088 o usa #PFMovil. pic.twitter.com/uenMTmZ6H5 — Policía Federal Mx (@PoliciaFedMx) January 9, 2019

Attempts to contact and request more information from the Mexican Federal Police were unsuccessful. U.S. Customs and Border Protection officials in Arizona are also unable to provide information because they furloughed their communication staff due to the ongoing partial government shutdown, which is now the longest in U.S. history.

Story continues

Effectiveness of walls under scrutiny

Trump's insistence on border-security funding is central to the fight that has resulted in the shutdown.

As part of his push to secure more funding, Trump cites high numbers of drugs coming across the Southwest U.S. border and claims that "These numbers will be DRASTICALLY REDUCED if we have a Wall!"

We lose 300 Americans a week, 90% of which comes through the Southern Border. These numbers will be DRASTICALLY REDUCED if we have a Wall! — Donald J. Trump (@realDonaldTrump) January 11, 2019

However, the vast majority of the drugs that he alludes to, including heroin, are increasingly caught at the legal ports of entry, which would be unaffected by the construction of additional physical barriers.

READ MORE: Most hard drugs seized at border ports; a wall wouldn't stop it

Critics routinely point to drug tunnels as a sign that walls don't work.

U.S. House Speaker Nancy Pelosi, D-Calif., broached the topic during one of several failed meetings between congressional Democratic leadership and Trump to try to end the stalemate.

As the president visited south Texas on Thursday, one of the border officials he met with explained how smugglers had turned to tunneling underneath areas of the border where physical barriers had been built.

"This is the second tunnel recently that we've located. This is an area that we actually have wall," Melissa Lucio, the Border Patrol's agent in charge of the McAllen station, told Trump as he looked on.

"We're doing such a great job in utilizing the right resources in that particular area, that they've (smugglers) become so frustrated they're using other tactics," she added. "They're actually digging tunnels. This is about 25 feet long, and 2 to 3 feet high."

Smugglers increasingly using tunnels

In this March 6, 2017 photo, a member of the Border

Tunnels have become an increasingly common tactic that smugglers have used to get around stricter enforcement at the border, especially in sections that have had physical barriers in place for many years and decades, such as Arizona and California's borders with Mexico.

Along the Arizona border, Nogales is one of the main areas where large numbers of drug tunnels are uncovered on either side.

Just last week, Mexican Federal Police located a 65-foot tunnel inside an abandoned business in Nogales, Sonora, about four blocks south of the international boundary, according to Mexican media.

Before then, on Dec. 17, Customs and Border Protection officials in Arizona said they and Mexican officials had located another tunnel in downtown Nogales as part of a "routine binational tunnel sweep."

The unfinished, 50-foot tunnel stretched nearly 44 feet into Arizona, from the drainage channels underneath the twin cities to an empty parking lot, CBP said.

READ MORE:

This article originally appeared on Arizona Republic: Amid border wall debate, third drug tunnel found in less than a month along Arizona border