Forty-six people were found dead Monday inside a tractor-trailer in a suspected migrant smuggling attempt on a Texas roadway, authorities said Monday night.

The victims were discovered inside the 18-wheeler on Quintana Road in San Antonio just before 6 p.m. local time, San Antonio Police Chief William McManus said.

Officials said 16 survivors — including four minors — were rushed to area hospitals for heatstroke- and dehydration- related conditions.

The discovery came after a nearby worker heard a cry for help and found the trailer with its doors partially open and bodies inside, police said.

First responders found one body outside the truck and dozens more inside.

“We’re not supposed to be able to open a truck and see stacks of bodies in there,” San Antonio Fire Chief Charles Hood said.

He added that there was no water inside and no working air conditioning or refrigeration.

Temperatures in San Antonio reached a high of 103 Monday amid an ongoing heat wave.

Three people were taken into custody in connection to the incident, but it was unclear what their involvement was.

The investigation has since been taken over by the Department of Homeland Security.

‘Mass casualty’ scene

Local journalists reported spotting yellow tarps near the back of the open truck, as well as a large “mass casualty — evacuation” ambulance rushing by and dozens of officers from the San Antonio Police Department, Homeland Security and Border Patrol on the scene.

The truck was abandoned in a remote area near railroad tracks.

Officers and first responders were walking the train tracks with thermal imaging cameras to find any possible survivors who may have fled, San Antonio’s KSAT news station reported.

All of the victims were believed to all have been undocumented migrants who crossed into the US illegally, according to The New York Times.

“The plight of migrants seeking refuge is always a humanitarian crisis but tonight we are dealing with a horrific human tragedy,” San Antonio Mayor Ron Nirenberg said.

Officers are investigating how the 46 people died, but heat is believed to be a key factor.

The death toll was initially reported at 20 people. The new count makes the tragedy one of the deadliest incidents of migrant trafficking in the US in recent years.

“We will continue to address the serious public safety threat posed by human smuggling organizations and their reckless disregard for the health and safety of those smuggled,” a spokesperson for US Immigration and Customs Enforcement said in a statement to the Post.