In the dim light of a vacant room, Manuel is fidgeting. In his hand are 25 blue pills — fentanyl, he says — the drug Mexican authorities are currently hunting down.
He could be jailed if he is caught with even one of them. "I don't want to know what will happen," he says, nervously.
Manuel — not his real name — is linked to the Sinaloa cartel, one of the world's most notorious traffickers of fentanyl, a deadly synthetic opioid 50 times more potent than heroin.
Over the past decade, the cartel has been making vast quantities of the drug to smuggle into the United States, fuelling an opioid epidemic that is killing tens of thousands of Americans a year.
But lately, that business has been under pressure, Manuel says.
"Right now, things is crazy, it's f***ed up," he says. "The business, what you hear is, yeah, it's been hit hard."
Since Donald Trump returned to the US presidency last year he has been heaping pressure on the Mexican government to stop the flow of fentanyl over the border, forcing Mexico's President Claudia Sheinbaum to launch one of the biggest crackdowns on the country's drug cartels in recent memory.
In just under a year since she was elected, Mexican authorities claim to have dismantled over 750 clandestine drug labs, arrested dozens of cartel bosses and seized massive quantities of fentanyl.
In February, under the threat of crippling tariffs from the new Trump administration, Sheinbaum agreed to send 10,000 Mexican National Guard troops to reinforce the US–Mexico border to "prevent drug trafficking, particularly fentanyl".
"There's a lot of surveillance, a lot of government, right? Both in the US and here in Mexico, and they're working together. And yeah, there's fear," says Manuel.
"I mean, it's completely at a standstill because of the laws that Trump is putting in place now. People are scared, you know."
The drug at the centre of US–Mexico tensions
For Mexico's cartels, fentanyl has proved a miracle product — cheap to make and easy to smuggle.
Unlike plant-based opioids like heroin, fentanyl does not require big plots of farmland or teams of labourers to cultivate a crop.
The chemical precursors for making it can be ordered in the mail from China, with enough to manufacture commercial quantities of pills arriving in a small package.
Cartel chemists can cook the drug in makeshift labs in home kitchens with very little in the way of specialist equipment.
The resulting product is extremely potent; even a single fentanyl pill can kill.
According to the US Drug Enforcement Administration (DEA), just 2 milligrams of the drug — about the size of a pencil tip — can be fatal, and some pills have been found to contain more than twice that lethal dose.
It is perhaps no surprise that Mexico's booming illicit fentanyl trade has become a point of political friction with its northern neighbour.
In the US, where an opioid addiction crisis has long been a problem, the surge of fentanyl from Mexico has helped fuel an unprecedented wave of drug overdoses.
Of the 107,000 overdose deaths in America in 2023, almost 70 per cent were caused by fentanyl and other opioids, according to DEA figures.
US intelligence agencies say most of the illicit fentanyl is coming over the border from Mexico.
Stopping fentanyl smuggling is now a "central" issue between the US and Mexico, according to Mexican academic and organised crime expert Guadalupe Correa-Cabrera.
"It's central because the United States government has made it central, blaming the cartels for a much more complex issue that the United States society is facing," she says.
Claudia Sheinbaum, a former scientist and mayor of Mexico City, was elected president in 2024 in a landslide victory, becoming the first woman to lead Mexico.
The 2024 election was the most violent in Mexico's history and public security was one of the top issues.
On taking office last October, Sheinbaum quickly broke with her predecessor, Andrés Manuel López Obrador's, softer approach to dealing with the cartels.
His so-called "hugs not bullets" strategy sought to avoid shootouts with Mexico's powerful and well-armed criminal networks, focusing instead on social programs to give young men an alternative to joining a cartel.
"This is the end of the 'hugs not bullets' strategy. That definitely failed," says Guadalupe Correa-Cabrera.
"We're back in the bullets time. We're back in the enforcement time. That didn't work either. That is the problem."
Trump's election in November 2024 only pushed Sheinbaum further down that path, she says.
The US president has continued to threaten military invasion unless Sheinbaum does more to curb fentanyl smuggling and crack down on the cartels, even accusing her of being "afraid" of them.
Sheinbaum has walked a delicate line to placate him, brushing off the taunts while pushing back on US threats to put boots on the ground in Mexico.
"We reject the presence of US troops because we are not in agreement, because Mexico is a sovereign, independent country," she said in response to a question from Foreign Correspondent at her daily press conference in Mexico City.
"We coordinate and collaborate with the United States. We do not subordinate ourselves."
"She manages Trump very well," says Correa-Cabrera of the Mexican president. "She confronts Trump, but not in a way that seems that it is a fight. She does it with respect."
But Correa-Cabrera warns that Sheinbaum is falling into a trap by adopting his war on fentanyl.
"Fentanyl is a discourse driven by the United States security agencies," she says. "It's a strategic interest for the United States to fight and to deal with the fentanyl crisis, to go after the cartels.
"And now Claudia is making that fight her fight. It's a response to the pressure exercised by Donald Trump."
Cracking down in a Sinaloa stronghold
That pressure is ever-present in the Mexican border town of Nogales, which sits up against the wall dividing Mexico from the US state of Arizona.
In recent months, local police officer Martin Pino has seen the arrival of more Mexican Army and National Guard units under Sheinbaum's crackdown.
On a routine patrol along the border, we are pulled over by a unit of heavily armed Mexican troops, who question Pino and our crew about what we are doing there.
Even local cops are not totally above suspicion these days.
It is not just the Mexican military presence that has been ramped up in Nogales — the US has also moved troops and military hardware to the US side of the border.
In one Nogales neighbourhood, where the houses sit right along the border fence, an armoured US Stryker vehicle now sits sentinel on a hilltop, just metres from Mexicans' backyards.
About a hundred Strykers have been deployed along the US border this year, each equipped with state-of-the-art cameras that can detect the slightest movement.
Pino says this one appeared in Nogales in recent months and has been keeping a watchful eye ever since.
Just beyond the outskirts of town, another Stryker sits parked overlooking a spot where the border fence is low enough to climb over.
[MAP Nogales]This might have been a blind spot in the past, but nothing goes unnoticed now.
Within minutes of the ABC's arrival, US Customs and Border Patrol agents arrive in a pick-up truck, followed by a detachment of US soldiers.
Pino is unfazed by the show of force on Mexico's doorstep. He welcomes the backup and says Mexican authorities are coordinating closely with their US counterparts.
"It's good because drug traffickers and people who traffic undocumented immigrants are slowed down and stopped when they see this type of situation with authorities on the US side," says Pinto. "We need all the help we can get."
Border crossings were already falling when Trump took office but have plunged further still, reaching the lowest level in decades this year.
But it remains to be seen how effective the crackdown can really be in a town where cartel power is deeply entrenched.
Nogales is a known Sinaloa stronghold.
Its proximity to the border has made it a prized possession for the cartel, which has long smuggled drugs and people into the US here, mostly through legal ports of entry.
Thousands of trucks, cars and pedestrians cross into Arizona through Nogales every day.
Last month, US Customs officers said they had intercepted more than a million fentanyl pills and other drugs in a series of seizures at the Nogales port of entry in a single week.
Still, Cartel smugglers have honed their methods of getting illicit goods into the US over the decades.
In the middle of town, right next to the border wall, Pino shows us one of their more ingenious methods.
Lifting a steel grate in the street, he climbs down a ladder into a broad, dark stormwater drain leading right under the border wall.
Cartel smugglers once dug tunnels from the backyards of nearby houses to join up with this main passage, smuggling migrants and drugs in secret right under the authorities' noses.
Half a dozen white patches are visible on the walls where authorities have sealed up cartel smuggling tunnels over the years.
"It's amazing, incredible," says Pino. "Even as a police officer, I find it hard to believe. It's like something out of a movie or some science fiction thing.
"There is so much power, so much money going into these activities to be able to do all the logistics, engineers, machinery to make this entrance to the tunnel. It's astounding."
This drain now needs to be constantly monitored.
Claudia Sheinbaum says her strategy for taking on the cartels is working, and points to a 40 per cent drop in seizures of fentanyl at the US-Mexico border since the start of the Trump presidency as proof.
But Guadalupe Correa-Cabrera says that number does not necessarily mean cartels are failing to get drugs into the US.
"If they go down, it's because probably they are entering without them knowing, because probably, you know, cartels are creating new tunnels or they are doing it through the sea," she says.
In June, US Border Patrol agents discovered a cross-border tunnel under construction between Tijuana and San Diego.
Running for almost a kilometre at a depth of up to 15 metres, they described it as "highly sophisticated".
Ms Correa-Cabrera fears Sheinbaum's focus on fentanyl risks coming at the expense of addressing the country's other issues, such as extortion, corruption and rising drug use, all to appease a US president who can easily change his mind.
"You have limited resources and those resources are being used by Claudia Sheinbaum to fight the wars of the United States," she says.
"This show is costing Mexico a lot of money … without addressing our own problems. It will backfire, not only on Claudia Sheinbaum, but on the Mexican people."
It is a criticism Claudia Sheinbaum rejects.
For now, she remains hugely popular in Mexico, enjoying an approval rating around 75 per cent, and is forging ahead with her cartel crackdown.
"Are we doing it to satisfy President Trump? No," Sheinbaum told Foreign Correspondent at her press conference in Mexico City.
"We do it because we don't want fentanyl to reach any young person in the United States, but we also don't want fentanyl to reach any young person in Mexico or any young person anywhere in the world."
Watch Mexico's Cartel Crackdown tonight on Foreign Correspondent at 8pm on ABC TV and iview.
