Mexican authorities said today that remnants of clothing and bones from six bodies have been excavated from a ranch on the desert outskirts of this border city where U.S. and Mexican officials are preparing for a protracted investigation of sites believed to contain numerous murder victims.

"There is a skull, some bones, some boots; there are bone fragments, including some that are small," Mexican Attorney General Jorge Madrazo said in a radio interview. "At this moment, nobody in the world could tell you whom they belong to."

After three days of exploring four sites where an informant alleged that as many as 100 bodies could be buried, law enforcement authorities from Mexico and the United States today said they have been unable to determine how many bodies may have been buried and said it could take weeks to determine the identities.

Mexican and U.S. law enforcement agents said the bodies are believed to be those of some of the nearly 200 Mexican and U.S. citizens who have disappeared from the border area around Ciudad Juarez and El Paso in the past five years. While law enforcement officials said most of those who disappeared were linked to drug trafficking, human rights organizations and associations representing the families of missing persons have alleged that unscrupulous Mexican police and military officials were involved in the crimes.

While law enforcement authorities started digging Monday following declarations that as many as 100 bodies could be buried in several graves, U.S. and Mexican authorities today backed away from concrete predictions.

"With respect to 100 bodies, that isn't an exact number," Jose Larrieta Carrasco, head of the Mexican attorney general's organized crime unit, which is heading the Mexican part of the investigation, told reporters outside the small ranch where officials recovered the remains. "We have not said 100 persons, or the remains of 100 persons, would be found in one place."

"The whole world is talking about a narco-cemetery filled with hundreds of bodies, and up until now, the authorities have not been able to confirm those facts," said Javier Benavides Gonzalez, chief of the Ciudad Juarez municipal police who said his officers are not participating in the federal investigation. "Unfortunately, there has been a lot of speculation."

One U.S. official said today it could take months to find all the bodies, and law enforcement authorities on both sides of the border said DNA tests and other identification methods could require weeks of work on each set of remains.

Meanwhile, a few teary-eyed relatives of people who have disappeared kept vigil around the barbed-wire fence surrounding the small ranch of pastel blue barns, a hacienda-style house and white rail fencing.

Throughout the day, vans, trucks and Mexican agents wearing black uniforms and ski masks to hide their identities entered the windswept compound where digging operations were underway.

At least two people identified as caretakers of the ranch, called Rancho de la Campana, have been sent to Mexico City for questioning by agents of the attorney general's office, according to Mexican newspapers. Authorities have not identified the property owners.

Although U.S. and Mexican authorities said they are investigating possible graves at three other remote locations, no remains have been found at those sites.

Rancho de la Campana is located on a well-traveled highway about 10 miles south of the city, but backs up to a high ridge of jagged rocks and sprawls across a desolate expanse of khaki-colored desert dotted by low scrub brush.

No animals have been seen on the property since the excavations began. Its only neighbors are an archery and shooting range on one side and a mechanic's shop across the road from the concrete wall that fronts the property's entrance.

The discoveries of the bodies, four of which were unearthed today and two on Tuesday, and the potential that dozens more bodies lie beneath the desert, have drawn international attention to one of the most violent areas of the 2,000-mile border between Mexico and the United States. Ciudad Juarez, a sprawling city of huge international assembly plants and shantytowns, is the headquarters of Mexico's most powerful drug mafia, the Juarez cartel.

Relatives and human rights groups point to the frequent disappearances of people of all ages and from all walks of life, and to Mexican and U.S. law enforcement's inability to resolve the crimes, as examples of the impunity that allows criminals and corrupt police to operate in Mexico.

"The thing is, in the Mexican context, you have investigation upon investigation, unfortunately leading nowhere," said Carlos Salinas, an Amnesty International official for Latin America. "We would hope this set of circumstances would . . . bring some kind of closure to these cases and hold these officials accountable."

But the international attention surrounding the excavations and the prominent role of the FBI and Drug Enforcement Administration has caused a political backlash in Mexico, where resentment runs deep against any U.S. involvement in Mexican affairs.

"We are risking that [the FBI and DEA] come to substitute" Mexico's police forces, Cuauhtemoc Cardenas, presidential candidate of a leftist party, told the Mexico City daily La Jornada. "We don't require any direct intervention from any police from anywhere."

Staff writer Lorraine Adams in Washington and researcher Garance Burke in Mexico City contributed to this report.

CAPTION: Leticia Lucero Medina, whose husband has been missing since July 1997, arranges some photographs of missing people.