LA JOLLA (CBS 8/CNS) - An arson investigation was underway Monday into four fires set at coffee carts on the UC San Diego campus.

The overnight fires at the La Jolla-area university complex caused about $5,000 damage per cart and another $2,000 damage to the contents, according to San Diego Fire-Rescue Department spokeswoman Monica Munoz. The total amount of damage is estimated at around $30,000.

Police told firefighters no one was in the area when one of the carts caught fire. Initial reports reveal that it's not clear if some kind of delayed device was used. At least three of the four carts are affiliated with Fairbanks Coffee. It isn't known if there's a connection.

Multiple people told CBS News 8 that this wasn't the first time the coffee carts have been vandalized. A janitor on campus said there have been several incidents in the past few weeks.

No injuries were reported.

University police and the city fire department were conducting a joint investigation, Munoz said.