At least 46 people were found dead inside a tractor-trailer in Texas on Monday, while another 16 have been hospitalised, and an investigation is underway to determine what happened.

According to local media outlet KSAT, the dead were discovered in an 18-wheeler near the 9600 block of Quintana Road, but at the time of writing this, it was unclear how the discovery was made.

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Subsequently, the San Antonio Police, along with fire trucks and ambulances, blocked a large section of the Quintana Road on the Southwest Side, while the US Border Patrol arrived on the scene at around 8pm, suggesting that those dead were likely to be migrants.

The outlet, citing SAPD officials, also reported that the death toll was expected to rise.

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The San Antonio Express, meanwhile, reported that the truck contained as many as 100 people. At the time of writing this, it was unclear whether the rest were safe.

Monday’s deaths are perhaps the deadliest incident involving migrants in Texas, where big rigs are often used to smuggle in illegal immigrants from the Mexico border. In 2017, 39 immigrants were found in a heated tractor trailer parked outside a Walmart in San Antonio, among whom 10 died.

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However, heat poses a severe threat to migrants attempting to cross over to the US in big rigs, as temperatures can rise severely within the crowded trailers.

On Monday, the weather in San Antonio was mostly cloudy, but temperatures still exceeded a 100 degrees Fahrenheit (37.7 degrees Celsius), something that may have played a part in the tragedy.