SAN ANTONIO, Texas (KXAN) — San Antonio Mayor Ron Nirenberg referred to it as a “horrific human tragedy.”

In an area on Quintana Road in southwest San Antonio, 46 migrants were found dead Monday inside the trailer of an 18-wheeler, and 16 others were taken to hospitals. Of the 16, four were teenagers, officials said.

The Associated Press reported Tuesday that Bexar County Judge Nelson Wolff said four others later died after being taken to hospitals, bringing the number of deaths to 51.

Wolff told AP that 39 men and 11 women are among the dead.

Mexico’s Secretary of Foreign Affairs Marcelo Ebrard said Tuesday at least 22 of the deceased were from Mexico. Another seven were from Guatemala, and at least two were from Honduras. Authorities are working to identify the others. He said Mexico is investigating alongside the U.S. Department of Homeland Security.

San Antonio Police Chief William McManus said three people are in police custody in connection with the deaths, but he’s still not sure exactly how they’re connected. He said a call came in at 5:50 p.m. that a worker nearby heard cries for help and saw the doors to the trailer partially open. The worker then opened the door and “found deceased individuals inside,” police said.

McManus said it was the “largest death toll due to human smuggling San Antonio has ever seen.”

The Department of Homeland Security is now leading the investigation.

Charles Hood, San Antonio Fire Department chief, said none of the 16 found alive at the time were able to get themselves out of the trailer — they all had to be helped out. He also said there were “no signs of water in the truck.”

“The patients were hot to the touch. Suffering from heatstroke, heat exhaustion,” he said. “They were too weak to get themselves out of the truck.”

Hood also said he had “no idea” how long the migrants were in the trailer. All 46 people found dead on scene were adults, he said. It was a “mix of males and females,” he said.

McManus said police are “not confident” everyone in the 18-wheeler was accounted for. He said dogs were searching wooded areas around where the 18-wheeler was found. That search will resume Tuesday morning.