Dating back 20 years, the rap sheet for a man known as Rafael Resendez-Ramirez notes convictions for aggravated assault, burglary and other felonies that have repeatedly put him in jail. But nothing in this long history of crime gave any warning of what he's allegedly done in the past few weeks.

Since the end of April, Resendez-Ramirez has allegedly murdered four people in Texas and two in Illinois, all of the crimes exceptionally brutal with no obvious motive. In response, the FBI this week rushed him onto its 10 Most Wanted list, both to attract publicity to the case and to warn potential victims.

"Something triggered him off into a killing spree," said a senior federal investigator. "We've seen it in other serial killers, and once someone starts killing like this, odds are he will kill again unless we get him first."

Resendez-Ramirez has used at least 30 aliases and numerous birth dates and Social Security numbers and has repeatedly changed his appearance, according to the FBI. He is thought to be 39 years old and to have been born in Puebla, Mexico. He enters the country illegally with apparent ease even though he has been deported or voluntarily returned to Mexico four times. His most distinctive feature is a snake tattoo on his left arm.

Though there is confusion about his identity, his alleged killings--which include two murders in 1997 and 1998 for which he is the chief suspect in addition to the six recent crimes--have several traits in common, investigators said.

"Clearly we have the train tracks," said Don K. Clark, special agent in charge of the FBI's Houston division, which is coordinating the nationwide manhunt.

Each of the eight murders ascribed to Resendez-Ramirez was committed close to train tracks, usually within a few hundred yards. Resendez-Ramirez is known to hitch freight trains in his almost constant wandering up and down the heart of the North American continent, from Mexico's central highlands, through Texas and the Mississippi River Valley to Canada.

"Also, we know in all of these murders, there have been instruments--firearms, blunt instruments or sharp instruments--used and the crime scenes have all been very violent," said Clark.

A few other patterns emerge from the mayhem ascribed to Resendez-Ramirez. He allegedly has repeatedly stolen victims' cars and driven relatively short distances to railroad crossings. He is known to sojourn at homeless shelters. All his alleged victims have been white and for the most part have lived in comfortable neighborhoods. He allegedly kills only at night, it seems, often after sneaking into a home.

His last known crime is typical.

On the night of June 14, a Monday, Resendez-Ramirez is believed to have jumped off a freight train that slowed or stopped on the outskirts of Gorham, Ill., a tiny river town in the southern part of the state. He allegedly made his way to a mobile home on a gravel road just 50 yards from the Union Pacific railroad tracks.

The owner of the home, George Morber, 79, was found the next day, dead of a shotgun wound to the head. His daughter, Carolyn Frederick, 52, lay near him, beaten to death. Morber's truck was found in Cairo, 60 miles to the south. Fingerprints link Resendez-Ramirez to the two killings, Illinois authorities said Monday when they charged him with the double murder.

In the Gorham killings, as well as in others in Kentucky and Texas, motive remains as much a mystery as the suspect's real name.

"If we get him alive, maybe, just maybe, we'll find out what drives him," said the senior investigator. "Otherwise, who knows?"

A task force of some 200 federal, state and local law enforcement officers in Houston is collating evidence from crime scenes and evaluating the tips that have inundated the FBI since Resendez-Ramirez made 10-most-wanted status on Monday. More than 1,150 calls have been made to an FBI hot line and hundreds of law enforcement officers across the country have responded to sightings of the suspect over the past few days.

On Tuesday, police stopped a freight train in Columbus, Ohio, and searched all 75 cars and the surrounding neighborhood with bloodhounds because a witness had called claiming to have seen a Hispanic man on board.

So far only one recent sighting of Resendez-Ramirez has been confirmed. According to the FBI, witness interviews and other evidence show that he spent June 16 to 18 in Louisville, where he apparently frequented a downtown homeless shelter.

The Railroad Killer

Following are slayings in which Rafael Resendez-Ramirez is a suspect. They all occurred near railroad tracks.

Fayette County

June 4, 1999

Gorham

June 15, 1999

(2 victims)

Weimar

April 30, 1999

(2 victims)

Lexington

Aug. 29,

1997

West University

Dec. 17, 1998

Houston

June 5, 1999

CAPTION: This poster circulated by the FBI features suspect Rafael Resendez-Ramirez, a Mexican national wanted for questioning in connection with five murders in Texas, one in Kentucky and two in Illinois.