Suspected serial killer Rafael Resendez-Ramirez was in the custody of U.S. Border Patrol agents and was let go just three weeks before he was put on the FBI's 10 Most Wanted list as a suspect in eight brutal murders.

Resendez-Ramirez, whose alleged trademark is to travel by hitching rides on railroad freight trains and kill near railroad tracks, was apprehended on June 2 by the Border Patrol after entering the United States illegally from Mexico and was subsequently sent back to Mexico, according to an Immigration and Naturalization Service statement. INS officials said the incident took place in Texas.

More than 1.5 million people a year are apprehended entering the country illegally and are sent back after processing that normally takes just a few hours. They are photographed and fingerprinted by the Border Patrol, and that information is entered into an INS database. The identities of illegal entrants are not routinely checked to determine whether they are wanted on criminal charges. The Border Patrol photograph was circulated nationwide by the FBI on Friday.

Resendez-Ramirez was added to the 10 Most Wanted list on Monday after he was charged in a double murder discovered June 15 in Gorham, Ill. He also is a suspect in six other killings committed in Kentucky and Texas over the past two years and is the subject of a manhunt involving hundreds of federal, state and local investigators.