By GINGER THOMPSON
 
LAREDO, Tex., Jan. 20 - Four months ago, Brenda Cisneros, a community college student, went across the border to Nuevo Laredo to celebrate her 23rd birthday with her best friend, Yvette Martinez, a 27-year-old mother of two small daughters.

They have been missing since.

Gerardo Contreras, 18, a construction worker from San Antonio and father of a small son, has been missing since May, when he went to the Mexican border town of Piedras Negras to attend his sister's baby shower.

Samuel and Gerardo Gonzalez, brothers who are 18 and 24, have been missing since December. Their mother, Rosita, said they last called home from a military checkpoint outside Nuevo Laredo as they returned from a trip to Monterrey.

"Mexican authorities do nothing, and the American authorities tell us that there is little they can do," said Priscilla Cisneros, Brenda's mother. "Most times we feel like we are fighting to find our children on our own."

Mexico's drug war has begun to move north of the border. In recent months, fighting among Mexico's most powerful cartels has spawned a wave of violence that at times has turned the streets into battlefields and plazas overtaken by gunmen firing grenades and assault weapons. Mexican law enforcement officials report a sharp rise in killings and kidnappings as cartel leaders struggle for control of this coveted corner of the border. American officials have warned that Mexican drug traffickers with false identification have taken up residence on the United States side of the border.

Nuevo Laredo's official figures said that 68 people were killed in the border town last year. However, Mexican law enforcement officials acknowledged that the real number was probably at least twice as high.

An F.B.I. agent assigned to Laredo said he believed that at least one person was killed in Nuevo Laredo each day and at least two people were kidnapped every month.

Often, the agent said, the kidnappings are carried out by municipal police officers who are secretly working for the drug traffickers. The officers pull their victims over for routine traffic violations and take them away. Michael Yoder, the United States consul in Nuevo Laredo, sounded alarms last month when he warned that the numbers of Americans kidnapped and killed had soared from about 3 a year to more than 25 in the last six months.

"We believe that local drug organizations have gone into the business of kidnapping for ransom," Mr. Yoder said. "For a long time, there was the assumption that people who stayed out of the drug business were safe. But that's not the case anymore."

Laredo and Nuevo Laredo, sister cities that are linked by a bridge across the Rio Grande, have long been coveted by drug cartels because more people and products move through Laredo than any other inland port in the hemisphere.

In recent years, the leaders of the Gulf Cartel, which controls the drug traffic through Laredo, have been killed or sent to prison. Some have continued their operations from behind bars. But new players have begun to fight for control.

Among the most ruthless is a group of former special forces officers known as the Zetas.

"There's a lot of paranoia among the Zetas right now," said an F.B.I. official who has spent most of the last 18 years working along the border. "They've got lookouts and safe houses all over Nuevo Laredo, and they're taking out anyone they suspect is working for their rivals. They're not taking any chances. It's really scary here right now."

Americans in other border cities have also reported being kidnapped. Dr. Charles Rogers, 57, a Brownsville oncologist who ran a cancer clinic in the Mexican city of Matamoros, which is just across the Rio Grande from Brownsville near where it feeds into the Gulf of Mexico, was abducted at his clinic and held hostage at gunpoint for several hours on Dec. 9, until he arranged for his wife to pay an $88,000 ransom.

In an interview, Dr. Rogers said he was stopped by three men who identified themselves as Mexican federal police officers. After the ransom was paid, he said, he jumped from a moving vehicle to escape because he was afraid the kidnappers were going to kill him. He left his clinic to Mexican physicians and now consults by videoconferencing.

"I have not been back since," he said. "I am never going back."

On Thursday, at least six prison guards and workers at the maximum security prison in Matamoros were found dead outside the prison. A week earlier, gunmen kidnapped about 25 people from a fishing village on the coast. Three of the victims, the former mayor of the village and his two sons, were killed.

The killing last year of a journalist for El Mañana, a Nuevo Laredo newspaper, has had a chilling effect on all news organizations.

"We censor ourselves," said Ramón Cantú Deandar, the editor of El Mañana. "The drug war is lost. We are alone. And I don't want to put anyone else at risk for a reality that is never going to change."

The families of the missing Americans said they had been oblivious to the violence in Mexico until their families were shattered by it. Many still seem paralyzed by fear.

A group of the relatives of victims at first seemed reluctant to speak during an interview. Most were afraid to have their photos taken or their names published because they worried about reprisals.

Many had not even reported their children missing to the Mexican authorities because they did not trust the Mexican justice system. Those who have filed missing persons reports said that rather than investigating those responsible for the kidnappings, the Mexican authorities accused the victims of being involved in the drug trade.

"It's their way of dismissing the cases," said Pablo Cisneros, a mechanic and Brenda Cisneros's father.

William Slemaker, a railroad conductor who is Yvette Martinez's stepfather, said: "I have told them that if my daughter was involved in drug trafficking, then prove it to me. But whether she was involved or not, they have a responsibility to investigate what happened to her."

Ms. Cisneros said: "I cannot tell you how painful this is. I can barely get out of bed in the morning."

Slowly, the relatives have found strength in each other and have begun to demand answers. They have posted photos of their missing sons and daughters on community bulletin boards and on the Internet at www.laredosmissing.com. They have given television interviews and sent letters to Washington and Mexico City. They have staged sit-ins at police stations and mayors' offices.

"Our lives are kind of on hold," said Iris Anzures, Jerry Contreras's older sister. She looked over at her baby daughter, born not long after her brother disappeared.

"I don't want to baptize her until Jerry comes back," she said. "And deep down, I don't know if he's ever coming back."

Ralph Blumenthal contributed reporting from Houston for this article.