SAN DIEGO -- Mass murders to send messages to rivals, daytime shootouts near schools and lucrative and brutal kidnappings are recent occurrences in Tijuana that are a growing concern to San Diego's FBI office.

"The violence of these kidnappings is beyond what I have ever seen in my career," says Keith Slotter, Special Agent in Charge.

The bloody war between rival drug cartels in Tijuana can be felt north of the U.S.-Mexico border.

"Some of the violence, some of the organizational aspects that exist in Tijuana and Baja, has spilled over into the U.S., especially the San Diego area," admits Slotter.

He says the cartels' drug trade into the U.S. has slowed over the last year, replaced with more human trafficking, extortions and kidnappings.

"The concern of more kidnappings of more victims being kidnapped here and then the victims being brought south is always a huge concern to us, perhaps our biggest concern in the kidnapping area," says Slotter.

This year, 15 U.S. citizens or legal residents have been kidnapped by gangs linked to the cartels. Some have been killed, and others were traded for ransom.

"The ransoms vary, and sometimes it is for a lot of money," Slotter says.

Those participating in the cartel violence and business also have ties to the U.S.

"You've got the Mexican Mafia, a very powerful gang organization both south of the border and here in California, especially in the San Diego area," says Slotter.

Other sources in law enforcement tell the I-Team that about half the recent cartel members murdered in Tijuana were deported by the U.S. government after spending time in U.S. prisons.

"So gang operations within various prisons in the U.S. are very strong, alive, and well," says Slotter.