At least 46 migrants were discovered dead Monday in and around a tractor-trailer abandoned on the side of the road on the outskirts of San Antonio, Texas.

The grim discovery was one of the worst disasters involving migrants in recent years, and it occurred five years after a similar deadly incident in the same central Texas city, just hours from the Mexican border.

"At this time we have processed approximately 46 bodies that have been triaged and tagged and declared deceased," San Antonio Fire Chief Charles Hood told reporters.

He stated that 16 people, 12 adults, and four children, had been transported to the hospital alive and conscious.

There were no initial details on the deceased's age or nationality.

"The patients that we saw were hot to the touch, they were suffering from heat stroke, heat exhaustion, no signs of water in the vehicle, it was a refrigerated tractor-trailer but there was no visible working A/C unit on that rig," Hood said.

Three people were detained as a result of the incident, according to authorities.

"Tonight we are dealing with a horrific human tragedy," San Antonio Mayor Ron Nirenberg told a press conference.

San Antonio, about 250 kilometers (150 miles) from the border, serves as a major transit point for people smugglers.

It has also been gripped by a recent heat wave, with temperatures in the area reaching 39.5 degrees Celsius (103 degrees Fahrenheit) on Monday.

A massive emergency operation involving police, firefighters, and ambulances was underway at the scene.

"These deaths are on Biden"

Texas Governor Greg Abbott, a Republican who advocates for tough immigration policies, blamed the disaster on President Joe Biden's "deadly open border policies."

"These deaths are on Biden," Abbott tweeted. "They show the deadly consequences of his refusal to enforce the law."

Read more: Biden under fire again over record US immigration

On his account, Mexico's foreign minister Marcelo Ebrard called the incident a "tragedy" and stated that the Mexican consul was on his way to the scene.

The nationalities of the victims have not yet been determined, according to Ebrard, but two Guatemalans are among the survivors.

The archbishop of San Antonio has also commented on the incident by saying: "once again, the lack of courage to deal with immigration reform is killing and destroying lives."

In 2017, a similar migrant tragedy occurred in San Antonio, when ten people died while traveling into the United States in a sweltering trailer with broken air conditioning and clogged ventilation holes.