US authorities are investigating reports that about 80 migrants may have been trapped in a tanker truck on Monday and were struggling to breathe, an official with Immigration and Customs Enforcement (ICE) said on Thursday.   

The official said ICE's Homeland Security Investigations unit was looking into 911 calls placed on Monday around San Antonio, Texas.

"We're dying," a male caller said in Spanish as he pleaded for help on one of the calls, according to recordings obtained by Reuters. Moans of others could be heard in the background.

The ICE investigative unit is seeking to learn if the incident may have been human trafficking, the official said.

"At face value, it's at least human smuggling," said the official, who requested anonymity to discuss the matter.

The investigation comes as arrests of migrants at the U.S.-Mexico border have been increasing after a steep drop at the start of the coronavirus pandemic. U.S. officials in January encountered nearly 78,000 migrants attempting to cross the southern border illegally or who were denied at ports of entry, a 6 percent increase over the previous month.

The Federal Bureau of Investigation field office in San Antonio and Bexar County Sheriff's Office did not immediately respond to requests for comment.

On one of the calls, a man said a white truck with approximately 80 people inside was stopped on the side of the highway, according to the recording.

"We don't have oxygen," the man said at one point.

Bexar County Sheriff Javier Salazar, whose jurisdiction includes San Antonio, described the scene in an interview with KABB, a local Fox affiliate.

"It's heartbreaking to hear because clearly behind the caller in the background, you can hear others in the car, in the trailer, and they're also screaming for help in Spanish and saying that they're out of air," he said.