Eleven suspected illegal immigrants whose badly decomposed bodies were found in a locked railroad car probably died slowly and painfully from severe overheating or asphyxiation, authorities said Tuesday.

The victims apparently boarded the grain hopper in Mexico four months ago and may have been smuggled into the country, said Jerry Heinauer, district director of the Immigration and Naturalization Service for Nebraska and Iowa.

Their nationalities were unknown and authorities said they did not know if the victims were men, women or children.

The car had been latched from the outside and there was no evidence of food or water inside, Sheriff Tom Hogan said. He said it was difficult to count the huddled bodies; authorities said there were as many as 11.

Authorities removed the bodies Tuesday, one day after they were discovered near Denison, 60 miles northwest of Omaha, Neb. The rail car was sealed and moved to Des Moines for examination by investigators.

The medical examiner will try to determine causes of death, and authorities will then begin trying to identify the remains.

Corinne Stern, the chief medical examiner in El Paso County, Texas, said the victims may have become delirious and suffered hallucinations, severe cramping, headaches and vomiting before succumbing to the heat or a lack of oxygen.

"They were probably subjected to temperatures equivalent to those inside a locked car during the heat of summer," said Stern, who is not involved in the investigation.

Heinauer said authorities did not yet know if the victims were being smuggled, but he said the case fits the pattern of some smuggling operations.

He said he was told by Mexican officials that the rail car left Matamoros, Mexico, in June. It had been parked long-term in Oklahoma since then, before being brought to Denison.

Julio Salinas, a supervisory agent with the U.S. Border Patrol at McAllen, Texas, said it is not unusual for immigrants to cross the border in rail cars. He said trains are often checked by Border Patrol and customs agents as they pass through cities.

"Most recently, a couple of months ago, we found 26 that had been inside a hopper car a couple of hours and some of them were dehydrated," Salinas said. "There were no fatalities. We do come across situations like that."

He said immigrants take life-threatening risks when they climb into a grain car on a smuggler's promise of freedom.

"There are some boxcars or grain hoppers that can only be opened from the outside," Salinas said. "Once they shut the door they leave their life in the hands of a smuggler."