"They were suffering from heat stroke, heat exhaustion," Hood said, later adding that there were no signs of water or working air conditioning unit on the semi-truck. 

The home countries of the immigrants and how long they were abandoned on the side of the road was not immediately known.

Hood said those still alive were too weak to help themselves out of the truck. Temperature highs in the San Antonio area were expected to reach the 90s to near 100 degrees Monday, according to the National Weather Service.

Three people have been taken into custody, McManus confirmed Monday, adding: "We don’t know if they are absolutely connected to this or not.

The investigation has been turned over to federal authorities, McManus said.

San Antonio Mayor Ron Nirenberg said the deaths were "nothing short of a horrific human tragedy."

Monday's deaths are the most recent incident officials believe to be related to border crossings. In March 2021, 13 people, including the driver, were killed when an SUV packed with at least 25 migrants, who authorities said entered the country illegally, crashed into a semi-truck in Southern California.

Months later, a van carrying 29 suspected migrants crashed in remote southern Texas, killing at least 10 people and injuring 20 others.

In years past, the number of crossings by undocumented migrants typically waned during summer months, as many avoided the lethal summer heat. Faced with U.S. policies that delay their entry and deteriorating conditions in their home countries, more asylum seekers have been trying to enter during summer, leading U.S. Border Patrol officials to issue warnings and train its agents to help keep migrants from perishing in the sun. 

“We know there’s been increasing volumes (of migrants crossing the border) over the past 6 months, 12 months,” Nirenberg said.

In May, border agents and officials encountered 239,416 migrants at the southwest border — up from 180,597 the previous May, according to U.S. Customs and Border Protection statistics released earlier this month. June numbers are still being compiled, but migrant shelter officials expect equally high numbers as last year, when more than 189,304 migrants crossed over, a two-decade high.

Texas Gov. Greg Abbott, a Republican running for reelection and critic of President Joe Biden's immigration policy, was quick to blame the deaths on the commander in chief.

"These deaths are on Biden. They are a result of his deadly open border policies," Abbott said on Twitter. "They show the deadly consequences of his refusal to enforce the law."

Rubén Minutti Zanatta, Consul General of Mexico in San Antonio, said in a tweet he was waiting outside San Antonio on Monday night waiting for federal authorities to release the nationalities of the deceased. 

Mexican Foreign Secretary Marcelo Ebrard said on Twitter that two of the 16 injured were Guatemalans.