Tijuana, Mexico ? The slayings of four young Americans in Tijuana sowed fear in Southern California on Friday as Mexican prosecutors tried to determine whether the youths were involved in the country’s violent drug trade or innocent victims of a brutal crime.

The victims, two men and two women in their teens and early 20s, said they were headed for a night of partying across the border only to be found strangled, stabbed and beaten a few days later.

Mexican officials are investigating whether any of the four San Diego-area victims had ties to the drug trade, after a toxicology report tested positive for cocaine on the body of Brianna Hernandez, who was either 18 or 19.

Another victim, Oscar Jorge Garcia, 23, was apprehended in the San Diego area in January 2008 with six illegal immigrants in the car, but never charged in the case, U.S. Immigration and Customs Enforcement spokeswoman Lauren Mack said.

The parents of 20-year-old victim Carmen Jimena Ramos Chavez on Friday described a vibrant Chula Vista High graduate who worked at an amusement park for children and planned to become a hair stylist.

“She was a happy girl, with a desire to explore the world,” said her father, Rogelio Ramos Camano, of Chula Vista. “Young people are like that. They think nothing will happen. I was like that, too.”

Mexican prosecutors said the victims had been bound and tortured — common tactics by Mexican drug gangs — before being left in a van in a dusty slum on the outskirts of Tijuana.

Jose Manuel Yepiz, a spokesman for the Baja California state prosecutor’s office, said investigators were examining a threatening letter to one of the victims from a jail inmate in San Diego.

Prosecutors said they had ruled out the possibility that the killings were a case of drug gangs targeting tourists.

Tijuana, which sits across the border from San Diego, has a reputation as one of Mexico’s most violent border cities. Authorities said 843 people were slain there in 2008, many in drug-related violence.