FRESNO, Calif. (AP) — As California struggles with a devastating drought, huge amounts of water are mysteriously vanishing from the Sacramento-San Joaquin Delta — and the prime suspects are farmers whose families have tilled fertile soil there for generations.

A state investigation was launched following complaints from two large agencies that supply water to arid farmland in the Central Valley and to millions of residents as far south as San Diego.

Delta farmers don't deny using as much water as they need. But they say they're not stealing it because their history of living at the water's edge gives them that right. Still, they have been asked to report how much water they're pumping and to prove their legal rights to it.

At issue is California's century-old water rights system that has been based on self-reporting and little oversight, historically giving senior water rights holders the ability to use as much water as they need, even in drought. Gov. Jerry Brown has said that if drought continues this system built into California's legal framework will probably need to be examined.

Delta farmer Rudy Mussi says he has senior water rights, putting him in line ahead of those with lower ranking, or junior, water rights.

"If there's surplus water, hey, I don't mind sharing it," Mussi said. "I don't want anybody with junior water rights leapfrogging my senior water rights just because they have more money and more political clout."

The fight pitting farmer against farmer is playing out in the Delta, the hub of the state's water system. With no indication of the drought easing, heightened attention is being placed on dwindling water throughout the state, which produces nearly half of the fruits, nuts and vegetables grown in the U.S.