SACRAMENTO  (AP) — A surge in thefts from mailboxes across the Central Valley last year victimized more than 2,100 people and sparked a federal crackdown that has led to charges against at least 27 people, authorities said Wednesday.

Postal investigators and local law enforcement officers targeted rings that were responsible for bulk mail thefts, particularly in the Sacramento, Fresno and Bakersfield areas, U.S. Attorney Ben Wagner said.

The thieves pried open boxes inside post office lobbies and multiple-slot postal lock boxes in many neighborhoods. They also broke into postal delivery trucks, in part so they could counterfeit postal-box keys.

They were after checks, credit cards, personal identifying information and prescription drugs delivered by mail. Losses topped $400,000.

More than a dozen postal inspectors and investigators were brought in from around the country in April to track the thieves.

Six people have been charged in Kern and Sacramento counties, while 21 others face federal charges. Nine of the federal defendants already have been sentenced, receiving up to four years in prison each. A conviction for theft of U.S. mail or possession of stolen mail carries a maximum penalty of five years in prison.

More charges are expected as the investigation continues, Wagner said during a news conference at the federal courthouse in Sacramento.

Mail theft happens nationwide and can victimize anyone, but the Central Valley seemed particularly vulnerable because of the widespread use of stand-alone neighborhood delivery boxes, Wagner said. He said those types of postal boxes are "tempting targets if you're a mail thief."

He also cited the region's long-time problems with the use of methamphetamine and other drugs. Addicts resort to stealing to fuel their habits.

"Where there's meth, there's mail (theft), and there is a correlation between those who are on substance abuse and going out to do things to accommodate their habit. And it just so happens that mail theft is one of those things," said Gregory Campbell Jr., deputy chief postal inspector for Western Field Operations, a region that includes California's Central Valley.

Copycat rings also popped up once criminals learned that the mail was easy to steal from the relatively lightweight lock boxes, which can be pried open with a large screwdriver or crowbar.

Sacramento and most of its surrounding suburbs were hit by two rings involving eight individuals who mostly struck neighborhood mailboxes, according to court records. Search warrants were used to recover hundreds of pieces of stolen mail and postal keys.

One woman involved in the Sacramento-area ring is charged with using her job as a retail sales clerk to create bogus credit card accounts using account information obtained from stolen mail.

Stolen checks can be counterfeited or "washed" with bleach to remove the name and substitute another, Wagner said.

"Washing the checks is sort of the old-fashioned, kind of low-brow school of bank fraud," Wagner said.

More sophisticated rings obtain large numbers of checks, open bank accounts under false names, then use the fake IDs to deposit and quickly withdraw the money.

The U.S. Postal Service delivers more than 150 million pieces of mail each day, Campbell said.

Last year, 16.6 million people, or 7 percent of all American adults, were victims of identity theft. But he said U.S. Department of Justice statistics show people are 2.5 times more likely to be victimized by a friend or family member and 22 times more likely to be victimized through a routine financial transaction than they are through mail theft.

While mail deliveries generally are safe, Campbell suggested that residents collect their mail as quickly as possible after it is delivered, check credit reports annually, create neighborhood watch groups and notify postal authorities if mail theft becomes a problem.

Where there are problems, the Postal Service can install more secure, but costly, neighborhood collection boxes or take other steps, including video surveillance, Campbell said.