AT least 46 people, believed to be undocumented migrants, were found dead Monday inside an 18-wheel tractor-trailer in San Antonio, a major city in south central U.S. state Texas, authorities said.

Sixteen others, including 12 adults and four children, have been transported to area hospitals, with at least five of them reportedly in critical condition, according to San Antonio police and fire officials.

The identity of the victims and their country of origin are not immediately clear.

Those who were found alive were hot to the touch and suffering from heat stroke and exhaustion, and there was no sign of water in the refrigerated tractor-trailer and no visible working air conditioning unit, San Antonio Fire Chief Charles Hood said. About 60 firefighters were on the scene.

Temperatures in San Antonio climbed to 101 degrees Fahrenheit (38.3 degrees Celsius) on Monday, while the heat in a packed trailer was likely way higher than that, according to the U.S. National Weather Service.

Three people are in police custody, but it’s unclear if they are involved in the tragedy, San Antonio Police Chief William McManus told a briefing. (Xinhua)