By Jim Forsyth

SAN ANTONIO, Texas (Reuters) - The truck driver 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, told investigators he was caught by surprise when he opened the trailer doors outside a Walmart store in San Antonio, only to be knocked down by a group of "Spanish" people pouring out of the rig, according to the criminal complaint filed in the case.

But the narrative attributed to Bradley, who could face the death penalty if convicted, was at odds with authorities' accounts of a small fleet of SUVs waiting in the Walmart lot to carry away some of the immigrants who clamored out of the truck.

Bradley was arrested on Sunday after police said they found the bodies of eight people in the truck, along with 30 to 40 others in and around the vehicle suffering from dehydration and heat stroke. All were illegal immigrants, the bulk of them Mexican nationals, ranging in age from 15 into their 20s and 30s, officials said.

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.

Daytime temperatures in the hours before the truck arrived had topped 100 Fahrenheit (37.8 Celsius).

Bradley 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 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 big-rig trailer, Homan said. But one of the survivors later told investigators that some 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.

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San Antonio Police Chief William McManus also said video footage showed several 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.

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 desperate conditions inside the 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's attention. Their pleas went unanswered until arriving at the Walmart, according to the account.

One survivor said about 70 people were already present when he climbed into the trailer with his group of nearly 30. Another estimated as many as 200 were aboard in total.

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 up the trailer, he noticed "bodies just lying on the floor like meat," the complaint said. 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 before heading on to San Antonio.

Authorities gave Bradley's residence as Clearwater, Florida. But Darnisha Rose, who lives in Louisville, Kentucky, and identified herself as his fiancee, told Reuters Bradley was a 47-year trucking veteran who made his home in his rig.

Rose described Bradley as a kind, family man whom she met two years ago when he was hospitalized in Louisville for a toe amputation and she was the housekeeper for his room. She said he recently had his right leg amputated.

Public defender Alfredo Villarreal, one of two lawyers representing Bradley, did not respond to a request for comment.

Mexico's foreign ministry said four of the 10 dead were Mexicans, as were 21 of the 29 people hospitalized. The Guatemalan government confirmed that one of its citizens was among the dead and two others had been found alive.

Crossing the border illegally from Mexico has long been a dangerous proposition, according the U.S. Customs and Border Protection, which has documented at least 7,000 deaths among those making the trek since 1998.

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, Sofia Menchu in Guatemala City, Steve Bittenbender in Louisville and Lizbeth Diaz in Mexico City; Writing by Steve Gorman; Editing by Frank McGurty, Jeffrey Benkoe and Lisa Shumaker)