Fifty-one undocumented migrants died after the tractor-trailer rig they were being smuggled in was abandoned Monday outside of San Antonio, Texas. Here is what we know about the tragedy:

- Who were the migrants? -

Officials at the scene intially put the toll at 46, and said 16 people, 12 adults and four children, were taken to hospital. The toll was later increased to 51, and media reports said several had died in hospital.

US officials did not give any breakdown by age, gender or nationality of the dead. Mexican President Andres Manuel Lopez Obrador said 22 were from Mexico, seven from Guatemala and two from Honduras. Lopez Obrador said he did not have information about the nationality of the others.

It is unclear whether there were other people who escaped earlier from the trailer.

- How were they found? -

The large tractor-trailer they were in was abandoned on the side of a road in a sparsely populated area about eight miles (14 kilometers) from the center of San Antonio. It was beside a railroad track and near several auto junkyards and car parts businesses.

It was also just a short distance from Interstate 35, which connects San Antonio with Laredo, a major hub on the Texas-Mexico border, 250 kilometers (150 miles) to the south.

San Antonio Police Chief William McManus said authorities were first alerted to the trailer by an emergency call at about 5:50 pm local time (2250 GMT).

He said a worker in a nearby building heard a cry for help and went to investigate. The worker found the trailer with its door partly open, and looked in to see a number of dead bodies.

- How did they die? -

San Antonio Fire Chief Charles Hood said the trailer was designed as a refrigerated space yet did not have a working air conditioning unit.

Sixteen people, including four children, were taken from the trailer alive apparently suffering from acute heatstroke and dehydration. The high temperature in San Antonio Monday was 103 degrees Fahrenheit (39.4 degrees Celsius).

According to Patterson, which makes cooling equipment for trailers, temperatures in hot weather can rise to as much as 150 degrees inside an uncooled, unvented trailer.

- Who's behind the tragedy? -

Still unknown. McManus said three people have been detained, but added that officials were not certain they were involved. Border officials say well-organized human smuggling rings run such operations, earning large sums of money from those wanting to sneak into the United States.

On June 14, 80 people from Mexico, Guatemala, Honduras, and El Salvador were discovered inside a tractor-trailer at a highway checkpoint north of Laredo, Texas. Three weeks earlier, agents intercepted a trailer with 48 people inside near Sierra Blanca, Texas.

President Joe Biden said the incident "underscores the need to go after the multi-billion-dollar criminal smuggling industry preying on migrants and leading to far too many innocent deaths."