Southern California, which law enforcement officials call the bank robbery capital of the world, got substantially tamer in the first half of this year, as the incidence of robberies plunged 37%.

A total of 461 robberies were committed in the 40,000-square-mile-area through June 30, according to the Federal Bureau of Investigation in Los Angeles. The decline puts the area in position to post a two-thirds reduction in bank robberies this year from the all-time record of 2,641 robberies in 1992, the year of the Los Angeles riots.

Bankers and law enforcement officials said the decline stems from substantial efforts in recent years to work more closely together to improve security and catch criminals.

"It's probably been the greatest cooperative effort since the cowboy days," said William R. Wipprecht, director of security for San Francisco- based Wells Fargo & Co.

According to John Hoos, a spokesman for the FBI's Los Angeles office, Southern California has more bank robberies than any other area in the country. He said the region accounts for about a quarter of all bank robberies in the United States.

One explanation for the high rate is Southern California's volatile mix of cars and bank branches. There are more than 3,500 bank branches in the region, giving robbers plenty of targets. The area's massive freeway system allows for quick getaways.

Gangs also contribute to the problem, officials say. With the Los Angeles riots, great numbers of guns got into gangs' hands, spurring a harrowing rise in "takeover robberies," where robbers actually hold bank staff and customers hostage, rather than just discreetly passing a note.

Law enforcement officials and banks credit the following steps with helping reduce the problem:

* Since the early 1990s, banks have gotten much better at advising each other on the effectiveness of various security measures, and working with police to position cameras so they take better pictures of robbers.

* Banks also have added bulletproof glass to hundreds of branches in the area, making robbers' guns less useful. Banks have increased their use of exploding "dye packs" hidden in cash; electronic tracers, and uniformed guards, who are often off-duty police officers.

* Banks have also come together through the California Bankers Association to start offering rewards for tips leading to the arrest and conviction of bank robbers.

In May, six banks, including Wells Fargo and Bank of America, contributed to a $40,000 reward given to an anonymous informant for information that led to the arrest and conviction of a robber known as "The Shootist," who would shoot his gun into a bank ceiling during a robbery. The Shootist had hit more than 50 banks in California, Texas, and Washington over an eight-year period.

Police, meanwhile, have infiltrated gangs, and arrested many of the most notorious robbers. Stiffer sentences have kept them in jail longer.

But the efforts have not come without problems. Earlier this month, Amir Brikho, of El Cahon, was reported to have been mistakenly given an exploding dye pack by a bank teller, after making a withdrawal from Valle de Oro Bank in La Mesa, Calif. The dye pack, which contained red ink and tear gas, exploded as Mr. Brikho walked to his car.

He then was reported to have taken off his shorts, which were burned, and to have run back into the bank branch, where he collapsed. He was treated at a hospital and released. Mr. Brikho said he has not yet decided whether to sue the bank.

Mr. Wipprecht acknowledged that there have been a handful of instances in which legitimate customers have been given exploding dye packs by mistake.

Two weeks ago, three robbers, carrying what turned out to be a fake bag of explosives, robbed a Bank of America branch in the Los Angeles suburb of Newhall of what was reported to be $750,000. A local newspaper said it was one of the largest robberies in Southern California in years. Law enforcement officials declined to comment on the size of the heist.

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