Masked bandits have attacked and robbed Baja California tourists at least seven times in recent months, acting with paramilitary precision and making off with cash and expensive vehicles, it was reported Saturday.

It was unclear whether the attacks represent a trend in the popular vacation area, but they have worried some surf club owners enough to cancel operations. In one case, a woman said she was sexually assaulted in front of her boyfriend and in another, a family was held at gunpoint.

U.S. authorities have long warned Americans to be careful when visiting Mexico because of threats ranging from kidnappings to drug-related violence.

"Criminals have been known to follow and harass U.S. citizens traveling in their vehicles, particularly in border areas," said an October warning issued by the U.S. State Department.

The recent attacks, though, have been particularly harrowing, involving carjackings by heavily armed men who held their victims hostage for as long as several hours. Most took place at night along a 190-mile stretch of coastline between Tijuana and San Quintin.

Lori Hoffman and her boyfriend, surf school owner Pat Weber, were robbed in October at a beach south of Ensenada within sight of 30 campsites. The couple evacuated their Encinitas home during recent wildfires and were in a recreational vehicle when they were attacked by two men wearing masks and combat boots.

The attackers shot up the RV when Weber initially refused to open the door and then terrorized the couple. Hoffman said she was sexually assaulted in front of her boyfriend before the men fled with $8,000 worth of laptop computers, jewelry, tools, and Weber's guitar.

"These guys were not novices," Hoffman said.

About two weeks ago, an El Cajon family of four was robbed by men they at first thought were police looking for a bribe.

Christopher and Debra Hall, their 16-year-old son and 21-year-old daughter were returning from taking part in the Baja 1000 off-road race when a car with flashing red lights and a siren pulled up behind them as they entered Tijuana. On previous trips, such stops had meant paying police a small sum to be let go.

"We weren't concerned at all. . . . You kind of expect it. It's part of the culture," Debra Hall said.

Instead, 10 men jumped out of two cars. Five got into their pickup truck and pointed guns at their heads. The men then drove the truck into isolated hills. They stole Hall's wedding ring and $1,000 in cash, along with other jewelry, a toolbox, and ransacked a 27-foot trailer.

In English, one man ordered the family to kneel. Hall said her son's face was shoved in the dirt.

"I thought he was going to get executed right there," she said.

She crawled over and covered him with her body.

"He was crying, and I was crying. I told him I loved him. He told me he loved me," she said.

The men eventually flung the family two sleeping bags and took off with the truck and trailer. The family eventually made it to a home where a woman phoned police, who took them to the U.S. border in San Ysidro. They crossed back into the United States wearing only flip-flops and the clothes on their backs.

The crimes have tarnished Baja's efforts to burnish its image and attract crucial tourist dollars.

Tijuana's new mayor, Jorge Ramos, has promised to create a special police force to patrol the coastal highway.

"Whatever it takes, it'll be done" to keep tourists safe, said Oscar Escobedo Carignan, Baja's new secretary of tourism.

"I don't want to downplay what happened," Carignan said. "We take it seriously. And we're making every effort to control it."