Four U.S. citizens, all from Southern California, were found dead in Tijuana, their bodies showing "signs of violence," Mexican and U.S. authorities said Thursday.

The two men and two women were found inside a car Saturday but their identities were not confirmed until Thursday. Their bodies were covered with blankets and had several wounds, the state prosecutor's office for Baja California said.

A spokesman for the U.S. Consulate in Tijuana confirmed the identities of the dead as Luis Games Chavez, 21; Oscar J. Garcia III, 23; Brianna Hernandez Aguilera, 19; and Carmen Ramos Chavez, 20. All were U.S. citizens and Southern California residents, the consulate spokesman said. He declined to give specific hometowns or say how long the four had been in Tijuana. Mexican authorities listed Ramos Chavez as a Mexican citizen.

The circumstances of the deaths were under investigation. No suspects were in custody, officials said.

Tijuana, once one of Mexico's most violent border cities, had experienced relative calm for much of this year, after top drug-trafficking figures were arrested in a military-led offensive. However, two weeks ago, seven police officers were killed in a single day in a series of brazen attacks that threatened to reignite the drug war.

Although a number of U.S. citizens have been killed in Mexico, they make up a small fraction of the thousands who die in drug-related violence in the country.