The grim discovery was one of the worst disasters involving migrants in the United States in recent years -- and came five years after a similar deadly incident in the same central Texas city, a few 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 said that 16 people had been transported to hospital alive and conscious -- 12 adults and four children.

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

"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.

Officials said three people were in custody over the incident.

The 46 immigants were presumed to be Mexican seeking to enter the US.