Mexican and U.S. authorities today discovered decaying remains in mass graves believed to contain scores of victims of drug-related violence near the border city of Ciudad Juarez, according to officials in both countries.

Mexican authorities have detained eight people in connection with the discoveries, Thomas J. Pickard, deputy director of the U.S. Federal Bureau of Investigation, said in Washington.

Pickard said at least 100 bodies are believed to be in two mass graves, which Mexican authorities said were located several miles south of Ciudad Juarez. More than 150 people have disappeared suspiciously in the last four years from Ciudad Juarez and from El Paso, Tex., located just across the border.

"This investigation is aimed at clearing up a series of drug-related murders and disappearances of Mexican and U.S. citizens, apparently by members of the so-called Juarez cartel," the Mexican attorney general's office said in a statement tonight.

However, Mexican human rights groups and organizations representing the families of missing persons have blamed corrupt Mexican police and military officials for many of the disappearances.

In numerous cases that have been exposed in recent years, Mexican police, agents of the attorney general's office and army officers and soldiers have been accused of protecting drug traffickers, their shipments and in some cases, participating in the killings of members of rival drug cartels.

The discovery of the mass graves comes as U.S. law enforcement authorities say Mexican traffickers are sending greater quantities and larger loads of drugs into the United States. Seizures of marijuana by U.S. agencies along the southwestern U.S. border, where 70 percent of all illicit drugs enter the country, were up as much as 33 percent over last year.

Hundreds of Mexican soldiers, assisted by agents from the U.S. Drug Enforcement Administration and FBI, worked throughout the day with bulldozers, shovels and infrared detection devices to uncover the bodies, according to Alfred Quijano, an editor of the daily Mexican newspaper El Norte.

In Washington, FBI Director Louis J. Freeh said that law enforcement officials in both countries were united in their efforts, and "a whole series of events would be unfolding over the next couple of days."

The FBI said it has dispatched about 100 agents and forensics experts to assist Mexican law enforcement officials and to identify the victims. Pickard said it appeared the bodies had been buried over an extended period, and 600 to 700 Mexican law enforcement officials were involved in the recovery effort.

Neither U.S. officials nor the Mexicans would say how they found the graves. Nor did they reveal the identities of the eight people arrested.

"We just started digging today," Pickard said. "Certain drug organizations may have utilized some of these places to bury individuals. We are working with Mexican authorities to recover the remains."

Ciudad Juarez, the headquarters and trafficking center for Mexico's most powerful drug cartel, is the most violent metropolitan area on the nearly 2,000-mile-long U.S.-Mexican border. The death of the Juarez cartel leader, Amado Carrillo Fuentes, after plastic surgery and liposuction two years ago, set off a burst of revenge killings as various factions within the cartel attempted to settle accounts. These shootings have tapered off in the last several months.

The Mexican attorney general's office tonight declined to speculate on how many bodies may be buried.

"At this time, the numbers are foggy," a spokesman for the Mexican government said. "With the investigation still running, it could be a couple of days before we find out exactly how many bodies are there."

Mexican army soldiers, along with DEA and FBI officials, began excavating the sites at 7 a.m., according to Quijano, the newspaper editor, who sent reporters and photographers to the scene.

"They have the place completely hermetically sealed," he said, adding that the Mexican army had confiscated film from his photographers.

The remains of the victims are being transported to El Paso where they will undergo forensic examinations in U.S. crime laboratories, according to the Mexican attorney general's office.

Mexican authorities have established toll-free numbers for Mexicans and U.S. citizens seeking information about relatives or others who have disappeared from Ciudad Juarez and El Paso in recent years.

If the graves prove to contain the remains of a large number of the missing, the findings could begin to solve some of Mexico's most gruesome crimes in recent years.

In addition to drug-related murders and disappearances, nearly 200 young women--many of them workers at U.S.-affiliated assembly plants in Ciudad Juarez-- have been killed and left in the deserts surrounding the city in the last several years.

Lorraine Adams and David A. Vise in Washington contributed to this report.