The gruesome murder of four young Americans in Tijuana remains unsolved, and Mexican authorities are investigating the possibility that the killings may be linked to the country's burgeoning illegal drug trade.

The bodies of the victims — who were reportedly beaten, stabbed and strangled — were found inside a van in eastern Tijuana on May 9, two days after at least two of them told relatives in Southern California that they were going to the border city for a night of partying.

Charles Smith, a spokesman for the U.S consulate in Tijuana, identified the victims as Luis Gamez Chaves, 21; Oscar Garcia III, 23; Brianna Hernandez, 19; and 20-year-old Carmen Jimena Ramos. All four were U.S. citizens from the San Diego-Chula Vista area.

Mexican prosecutors told the Associated Press they had ruled out the possibility that the killings were a case of drug cartels targeting tourists. Investigators for the Baja California state prosecutor's office are reportedly examining a threatening letter to one of the victims sent from a jail inmate in San Diego.

Mexican officials said a toxicology report on Hernandez's body tested positive for cocaine, the AP reported. Another victim, Garcia, was apprehended in the San Diego area with six illegal immigrants in a car, but he was not charged, U.S. Immigration and Customs Enforcement spokesman Lauren Mack confirmed to FOXNews.com.

No arrests have been made and no motive has been identified. FBI spokesman Darrell Foxworth said bureau officials in San Diego were providing assistance to Mexican authorities. He declined to elaborate, citing an ongoing investigation.

"If they have any information about a motive, they haven't shared it with us," said Chula Vista Police Department Public Information Officer Bernard Gonzales, referring to officials at the Baja California state attorney's office.

Attempts to reach the Baja California state prosecutor's office were unsuccessful on Monday.

Hernandez and Ramos were reported missing by their parents just before midnight on May 8, Gonzales told FOXNews.com.

"They indicated that they always answered their cell phones and answered them regardless of the situation, and that they were no longer answering," Gonzales said. "The parents had indicated there was a possibility that they went to Tijuana to a number of nightclubs popular with young people. So knowing the possibility that they had gone to Tijuana, the parents were worried."

The victims' bodies were found a day later.

"Unfortunately for the families, the worst news they could've gotten," Gonzales said. "At this point, it appears they went over on their own volition and were met with a bad end."

In February, the U.S. State Department issued an alert to travelers, warning them of increased homicide, petty theft and carjackings in Mexico, particularly along its northern border with the U.S.

"These conditions are widely known and reported on in Mexico, as well as in the U.S. border region, but many tourists and business people are less aware," the advisory read.

Ramos' brother-in-law, who declined to be identified due to fears of retribution, said the outgoing Chula Vista High graduate wanted to become a hairstylist. She and Hernandez were best friends, he said.

"She was just a normal, regular 20-year-old girl," he told FOXNews.com. "She liked going to beauty school, hanging out with her friends, dancing, that type of stuff."

He said Ramos' parents were very distraught. "They're hurting right now," he said.

Ted Galen Carpenter, vice president for defense and foreign policy at the Cato Institute, a Washington-based think tank, said drug traffickers in and around Tijuana remain "aggressive enough" that no nearby area is entirely safe.

Referring to the warring Sinaloa and Arrellano Felix drug cartels in the region, he said, "If either one of those cartels believed that these individuals believed that they were somehow connected to the other cartel, they signed they own death warrants by traveling there."

Carpenter, who recently authored a study entitled "Troubled Neighbor: Mexico's Drug Violence Poses a Threat to the United States," said it has become a "high-risk adventure" to travel to Mexico, particularly in Tijuana, where 843 people were killed last year, primarily due to drug-related violence.

"Unless there's a significant improvement in the security situation, these incidents are likely to persist and perhaps increase, especially as the Mexican economy suffers more and more from the global recession," he said. "It drives more and more people to be receptive to offers from the cartels. People who are perceived to be rich Americans are going to be targets."

Chula Vista Police Lt. Scott Arsenault warned against "kids" crossing the border into Mexico and said the victims most likely "ran into the wrong people."

"How often do you have a quadruple murder? If that happened here, there would be an outrage," Arsenault told the San Diego Union-Tribune.