Four days after Mexican and U.S. law enforcement agents began searching for bodies buried on a ranch south of this border city, virtually nothing is known about the lives of the six victims whose remains have been unearthed. But authorities are slowly piecing together information on the way they may have died.

"We presume [they were killed] by firearms, but we found them bound around the skull and mouth, so there could be indications they were suffocated," Mexican Attorney General Jorge Madrazo said today after touring the small, dusty ranch where the remains were found this week.

The bodies were buried in a hole shielded from a busy highway by two sky-blue barns and a barbed wire-topped cinder block wall. Abandoned cars riddled with bullet holes were found nearby.

Investigators believe that at least three of the victims were men over age 50, all of whom were wearing blue jeans. Two had on cowboy boots; the third wore sneakers. Authorities said they could have been killed and tossed into the grave as long as five years ago.

Mexican and U.S. officials said they believe the victims were killed by the Juarez cartel, Mexico's dominant drug mafia, but human rights organizations and relatives representing nearly 200 people who have disappeared from Ciudad Juarez and neighboring El Paso, Tex., in the past five years allege that corrupt Mexican police and soldiers were involved.

Authorities today allowed reporters, who have besieged this violence-prone city since the excavations became public on Monday, to tour Rancho de la Campana for the first time since the remains were found.

Although FBI officials estimated as many as 100 bodies could be found on this ranch and three other locations near Ciudad Juarez, Director Louis J. Freeh said a source led agents "exactly to the site where the six remains were located." Freeh would not speculate on whether more bodies would be found. No bodies have been found in the past two days.

Top U.S. and Mexican law enforcement officials held a news conference under a wind-battered tent on the ranch today to defend their highly publicized investigation and to attempt to blunt mounting criticism by Mexican politicians and news media of U.S. involvement in the criminal probe.

"I'm not selling out my country!" Madrazo shouted into his microphone in response to a question from a Mexican television reporter. "Cooperation does not harm sovereignty. What hurts it is drug trafficking. . . . I am not going to leave any corner of this country to the sovereignty of drug traffickers."

Freeh told reporters that the FBI "will work as hard as we can and as long as we have to" to complete the investigation. He said information about the locations of possible graves had been received by the Mexican and U.S. governments and "comes from more than one source, in fact, several sources."

But Freeh said that the ongoing excavations were largely the result of information from a source who only recently came forward with data that was "very well corroborated." Other FBI officials have said the source was a former Mexican police official who claimed to have witnessed the burials.

Families of missing Mexican and U.S. citizens have consistently criticized both governments for failing to aggressively investigate the disappearances.