NUEVO LAREDO, Mexico — Brenda Cisneros, 23, kissed her father goodbye after dinner in Laredo, Texas, just after 11 p.m. on Sept. 17. It was her birthday, and she was headed with a friend, Yvette Martinez, for a late-night concert across the border. The two drove across the international bridge into this sprawling town, famous for dancing and drinking spots. They never returned home.

Jerry Contreras, 17, left San Antonio one day last May and drove across the border into Piedras Negras to attend a baby shower. There, witnesses said, he became involved in a minor accident with a gold SUV, whose enraged driver rammed Contreras' Ford Escort, followed him to the party and threatened him. Contreras ran and hid in a grocery store, but several armed men dragged him out. He has not been seen since.

Cisneros, Martinez and Contreras are now listed among the dramatically increased number of U.S. citizens who have recently been reported missing or kidnapped along the border, especially around Nuevo Laredo. Last month, U.S. consular officials here issued a warning to the thousands of Americans who cross the bridge each week, including Mexican Americans visiting relatives or shopping and tourists on short sightseeing trips.

One U.S. official said that while some of the missing appear to have been innocent victims, more were probably involved with drug traffickers. In either case, "no one deserves to be kidnapped, tortured or killed," said the official, who spoke on condition of anonymity.

"We're seeing outright lawlessness in Nuevo Laredo. Things are just getting out of hand," the official said.

Law-enforcement officials said some disappearances may be related to a war among Mexican drug cartels vying for control of the crossing. It is the busiest commercial gateway on the 2,000-mile border, and millions of dollars' worth of marijuana, cocaine and heroin are smuggled north by truck and train among cargoes of legitimate goods.

Michael Yoder, the U.S. consul here, said one Mexican drug gang called the Zetas, composed of former military commandos who deserted from the Mexican army, has reportedly gone into the business of kidnapping for ransom, and an FBI official said he believed that drug gangs sometimes used kidnappings to raise money after a business setback such as a major drug bust.

Yoder said 27 U.S. citizens have now been reported kidnapped or vanished in the Nuevo Laredo area since August, 15 of whom are still missing. In previous years, he said, three or four cases would be reported, on average.

He declined to release details, but local news media reported that one victim was found shot in the back of the head, execution-style.

The FBI official said some of those returned alive had been held captive for days or even months after their abductors demanded ransoms as high as $100,000.

While some victims' families remain silent out of fear, others have started to come forward. The families of Cisneros and Martinez recently created a Web site — www.laredosmissing.com — to draw more attention to the kidnappings.

Until recently, drug-related violence was generally confined to the Mexican side of the border. Now, officials say, it is directly affecting the United States, both in higher numbers of incidents and in more brazen attacks.

In December 2003, grenades were thrown into a Laredo home, and the FBI said it believed the Zetas were involved. Last weekend, Laredo firefighters found the bodies of two young men, with hands bound, in the trunk of a car that had been set ablaze.

The rise in violence has caused alarm in both Mexico City and Washington. About 40 percent of the goods passing from Mexico into the United States cross the border here by truck or rail, and many worry that the problem could hurt legitimate commerce.

Twice last year, the Mexican government sent soldiers to patrol the streets of Nuevo Laredo. Traffickers are armed with AK-47 assault rifles, grenade launchers and bazookas, outgunning and intimidating local police, and 11 local officers have been killed since 2002.

"They want to show force, want to be seen," said Nuevo Laredo Mayor Daniel Peqa Treviqo. "On the U.S. side, they have learned it is better not to be seen."

One evening last week, relatives of Martinez and Cisneros sat at a kitchen table in Laredo and wept. They complained of being caught in a legal "no-man's land," saying that U.S. officials have no authority to investigate crime in Mexico and that Mexican authorities are either corrupt or intimidated by drug gangs.

Cisneros' father, who spoke on condition his name not be used, recalled his last words to his daughter as she left her birthday dinner: "I know you're 23 now, but you're still my baby. Be careful."