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Criminals have been smuggling migrants across the Mexico border into the US in everything from oil tankers to refrigeration trucks for years — expertly “cloning’’ the vehicles to look like legal transport.

Monday’s horror in San Antonio, Texas — which led to the deaths of at least 53 immigrants stuffed into a sweltering 18-wheeler “cloned’’ to look like a legally registered rig — only highlighted the tragic situation, an expert told The Post.

“It took this many people to die so we can be talking about human smuggling again and immigration,’’ said former Deputy Special Agent in Charge Harry Jimenez of the San Antonio Immigration and Customs Enforcement Office.

Jimenez shared photos of past investigations he worked on where an empty oil tanker — and even a lookalike Border Patrol vehicle — were used to ferry migrants inside them. Oil trucks in Texas can often bypass Border Patrol checkpoints by traveling off the interstate, Jimenez explained.

In an even more brazen cloning attempt, smugglers tried to outsmart the Border Patrol by pretending to be one of them. A smuggling organization copied a Border Patrol vehicle down to faking a federal license plate.

“There’s a lot of money being made smuggling people in the US,” Jimenez said. “When you have organizations that are going to these lengths of cloning vehicles and going across the US, they are very sophisticated.

“We’re talking about billions of dollars that exchanged hands in human smuggling fees. The only thing that’s making more money than human smuggling is smuggling drugs.

“It hardly ever makes national news,’’ he said. “The majority of the time, Border Patrol or the local law enforcement is able to rescue these [immigrants] before there are this many dead.”

Tragically, the driver of Monday’s rig was able to get past a Texas border checkpoint and make it to San Antonio before his truck apparently developed mechanical problems and he abandoned it on the side of the road.

By that time, the truck had turned into a coffin amid 103-degree temperatures, leaving “stacks of bodies’’ inside.

San Antonio is a smuggling hub, Jimenez said.

It is the first major city closest to the US-Mexico border and sits at the crossroads of Interstate 35, which goes all the way north to Canada, and Interstate 10, which takes drivers from California to Florida.

Before smuggled illegal immigrants arrive in San Antonio, they have changed hands between several smuggling organizations, depending on their country of origin, to get to the border. Once they are illegally crossed into Texas, they pay smugglers to take them to a “stash house” in a border town such as Del Rio or Laredo, Texas.

“You can have anywhere from 20 to 100 in small, one- or two-bedroom, pre-fabricated trailers or small houses with one bathroom,’’ Jimenez said of the migrants. “They’re all sleeping on the floor. There’s another person in charge of watching the stash house to make sure that no other organization steals the undocumented [immigrants].

“Sometimes they bring them food. The majority of the time, they’re moving them from one house to another.”

From the stash houses, the immigrants are moved to a tractor-trailer in the cover of night. The vehicles often come from Laredo, Texas, he said.

“You’re talking about Laredo where more than 20,000 trucks cross [the international border] every day. You have hundreds of warehouses, so it’s not out of place for a truck to be driving in the middle of the night,” Jimenez said.

The big rig then makes the journey north on Interstate 35 to San Antonio, which may or may not be their final destination. In San Antonio, another smuggler is waiting to take them to cities across the US.

“This happens a lot. There are several cases where the Border Patrol was able to identify tractor-trailers with people from 50 to 200 people in the back. That’s a weekly event,” Jimenez said.

Jimenez, who led smuggling investigations for 14 years in San Antonio, said the public doesn’t typically hear about these cases because the smugglers are able to get immigrants to their destination safely or because the bodies are dumped somewhere.

In the latest case in San Antonio, the tractor-trailer had been designed to look like a legally registered truck elsewhere in Texas.

“If the vehicle was cloned, what does it tell us: First, that it’s not the first time they did it — this is a very organized human smuggling venture,” he said.

Because of the amount of money that is at stake, Jimenez said he believes smuggling will never stop.

“Any politician that is trying to make these deaths an issue, I will call them out,” Jimenez said. “These are human beings that died. We need to fix the problem, we need to have an honest conversation about border security and immigration reform. Until that happens, this will continue to happen.”