The BBC's Jason Mills: "The big worry is that Resendez-Ramirez will strike again"

But it has emerged that the suspect, Rafael Resendez-Ramirez, was held in custody and deported to Mexico by the US immigration officials, as recently as 1 June.

The 39-year-old Mexican, a heavily-tattooed ex-convict, is wanted for questioning in connection with five killings in Texas, one in Kentucky and another two in Illinois.

Four of the eight murders - two in Texas and the two in Illinois - occurred after Resendez-Ramirez was deported.

Police in Lexington, Kentucky search campsites close to a railway line

It is thought that he returned to the US shortly afterwards and is now travelling across the Midwest on freight trains.

Federal Bureau of Investigation special agent Don Clark, heading the operation, did not disclose how the suspect slipped through the authorities fingers.

"I don't think it's a blow to our investigation," he said.

"Clearly we would like to have him right this minute, but what it does do is give us the most current photograph we can get out to the public."

Railway connection

All eight killings took place near railway tracks, and most of the victims were bludgeoned to death.

Fingerprints have tied Resendez-Ramirez to several of the murders, and he faces the death penalty if caught and convicted in Texas.

The FBI say he was last seen in Louisville, Kentucky, a week ago.

But it is thought he may be heading towards El Paso, Texas, and the Mexican border, where he has relatives.

On Monday, the FBI placed Resendez-Ramirez on its Ten Most Wanted list, describing him as "extremely dangerous" and offering up to $50,000 for information leading to his arrest.

Agent Clark said: "We're dealing with a transient, and it is going to take good police hard work to find him."

Twenty aliases

He said Resendez-Ramirez, a migrant worker, had used about 20 different aliases, five dates of birth and three birthplaces.

Arrested on 1 June and released

Sheriff Rick Vandel said: "He is a lot more clever than we first thought. He can change his appearance with different eyeglass frames and wigs. He'll shave his moustache and cut his hair."

FBI chief Louis Freeh said they had been deluged with calls: "A lot of sightings are inaccurate, but we appreciate that in the sense that it indicates a willingness by people to help us."

Elderly victim

The latest victims were 75-year-old George Mober and his daughter, Carolyn Frederick, 51, who were found dead in a mobile home in the small Illinois town of Gorham on 15 June.

On Monday, Gorham police charged Resendez-Ramirez with the killings after his fingerprints were found at the crime scene, which was only 50 yards from a railway line.

The suspect's killing spree began in Lexington, Kentucky in August 1997.

He has an extensive criminal record including convictions for weapons offences, burglary, rape, assault and vehicle theft.