HOUSTON (AP)  Three people are charged with human smuggling after a dozen more people believed from Latin America were rescued from a locked box truck where they”d been left to swelter in the Houston summer heat, Harris County authorities said Monday.

Officials said the victims spent hours banging on the inside walls of the truck trying to obtain help.

Joanne Musick, chief of the sex crimes division of the Harris County District Attorney”s office, said an alert Houston police officer, Chris Meade, made the discovery Sunday while working an unrelated case, spotting the rental truck in a construction site parking area where it was apparent it should not have been.

“Things just did not appear correct to him,” Musick said.

She said the presence of Meade and other officers then spurred some activity near the truck “and really drew his attention.”

When he opened the latch of the truck”s cargo box, the dozen people inside appeared drenched in sweat, with no food and little water and “looked like they were on the verge of heat exhaustion,” she said.

“He was able to realize these were undocumented immigrants that were being potentially smuggled or trafficked into the country,” Musick said.

Department of Homeland Security officers and other Houston police were summoned and in the course of interviews three people at the site were detained.

Priscila Perez Beltran, 21, Adela Alvarez, 26, and Nelson Cortes Garcia, 27, subsequently were arrested on three human smuggling charges and were scheduled to appear in court Tuesday. They were each being held on $300,000 bond.

Musick said the charges were being enhanced because of the substantial likelihood of bodily injury or death faced by the victims in the truck. Temperatures in Houston on Sunday were in the 90s and topped 100 degrees inside the truck.

“There is no good season for human trafficking, but summer time in Houston has to be the worst,” Harris County First Assistant District Attorney Tom Berg said. “Thirty more minutes and this could have been a dozen homicide cases.”

It was not immediately certain if the three charged are part of a larger smuggling ring. The case remains under investigation, authorities said. They three face up to 20 years in prison if convicted.

Homeland Security officials said the disposition of the 12 victims in the case also was not immediately certain since they could be needed as material witnesses to testify if the case goes to trial. Their exact home countries had not yet been determined but authorities said they were believed from Mexico, Guatemala and El Salvador.

They were given food and water, were checked out by emergency medical teams and interviewed.

“Each of those people had themselves paid or someone on their behalf paid to get them into the United States,” Musick said.

Musick said authorities hadn”t determined yet if they were headed for use as laborers or for sex trafficking but said historically they are “exploited in a number of ways.”

Houston is a little more than 300 miles (482.78 kilometers) northeast of the Mexican border.