DES MOINES, Iowa -- 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 at least as many as 11 in the railcar.

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.