By Jim Forsyth

SAN ANTONIO, Texas (Reuters) - The man accused of smuggling at least 100 illegal immigrants inside a sweltering tractor-trailer, 10 of whom died, has said he was unaware of the human cargo he was hauling until he took a rest stop in Texas, court papers showed on Monday.

James Bradley Jr., 60, was arrested on Sunday after police said they discovered dozens of undocumented Mexican and Guatemalan nationals, some unconscious in the back of the truck, others staggering around the vehicle in the parking lot of a Walmart store in San Antonio.

Authorities called to the scene found the bodies of eight illegal immigrants, along with 30 to 40 others who survived the ordeal but were suffering from dehydration and heat stroke, some of them as young as 15.

Two died later, bringing the death toll to 10, while 29 remained hospitalized on Monday, according to Thomas Homan, acting director of the U.S. Immigration and Customs Enforcement (ICE). Outside temperatures topped 100 Fahrenheit (37.8 Celsius) on Sunday.

Bradley, from Clearwater, Florida, made a brief appearance in federal court on Monday in San Antonio, where he was charged with one count of transporting illegal immigrants - a felony offense for which he could face capital punishment if convicted because the crime resulted in deaths.

More than 100 people were originally crammed into the stifling trailer of the big-rig, Homan said. But one of the survivors later told investigators that some immigrants managed to flee the scene before police arrived, swarming out of the truck when the rear doors opened to be whisked away by six black sport utility vehicles waiting for them nearby.

San Antonio Police Chief William McManus said video footage showed several other vehicles coming to pick up people who were inside the truck, though Bradley, according to court documents unsealed on Monday, denied seeing any such vehicles.

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Two of the survivors, according to the criminal complaint, recounted having been smuggled in small groups of immigrants across the Rio Grande River from Mexico to Texas, where they were harbored in "stash" houses around the border town of Laredo before being rounded up into the tractor-trailer for the trip to San Antonio, about 150 miles (240 km) to the north.

Describing unbearable conditions inside the crowded, pitch-black interior of the truck without water or proper ventilation, one survivor recalled people taking turns to gasp for fresh air through a hole in the trailer's side. Some passed out, while others shouted and pounded on the walls of the truck to get the driver to stop. Their pleas went unanswered until arriving at the Walmart, according to the account.

One survivor said 70 people were already inside when he got on, while another estimated the total at roughly 180 to 200 people. One said they were told they would have to pay smugglers $5,500 after arriving in San Antonio.

Bradley told investigators he did not know anyone was inside the truck until he parked near the store to use the bathroom and heard banging and shaking coming from the back, according to the criminal complaint.

When the driver opened the back doors of the trailer, he noticed "bodies just lying on the floor like meat," according to the interview summary. Some 30 or 40 people got out and "scattered," Bradley told investigators.

According to the complaint, Bradley told investigators he was hauling the trailer from Iowa to Brownsville, Texas, to deliver it to its new owner. He said he had stopped in Laredo to get the vehicle washed and polished before heading on to San Antonio.

After discovering the human cargo he claimed to have been unwittingly carrying in his trailer, Bradley said he called his wife but did not call 911 for emergency services, according to the court documents. Bradley also said he did not see any SUVs or other vehicles waiting in the parking lot where he stopped.

Pyle Transportation, which owned the truck, confirmed it was sold a month ago and that Bradley was hired to take it to Brownsville, according to a person answering the telephone at the company, who declined to give her name.

Alfredo Villarreal, an assistant federal public defender and one of two lawyers representing Bradley, did not respond to a request for comment.

Mexico's government said its consul general in San Antonio was working to identify the victims' nationalities. The Guatemalan government confirmed that some of the victims were from Guatemala.

Crossing the border from Mexico has long been a dangerous proposition, according the U.S. Customs and Border Protection. Since 1998, the total number of documented deaths stands at more than 7,000, with peaks in 2005 and 2012. Even those numbers could understate the risks, as Border Patrol said it only tallies the deaths when the agency finds the bodies, and the figures may exclude some deaths reported to local authorities.

In what is considered the worst illegal immigrant smuggling case in U.S. history, 19 people died after traveling in an 18-wheeler truck through Victoria, Texas, in 2003.

(Additional reporting by Barbara Goldberg and Jonathan Allen in New York, Letitia Stein in Tampa and Sofia Menchu in Guatemala City; Editing by Frank McGurty, Jeffrey Benkoe and Lisa Shumaker)