The gang

L.A. County Sheriff Lee Baca and detectives from the Major Crimes Bureau detail their investigation into five men arrested in April 2013 in a series of rooftop bank heists. (Irfan Khan / Los Angeles Times)

The men had their pick of eight banks at the busy intersection of Diamond Bar Boulevard and Grand Avenue. They chose the Citibank branch that used to be a Blockbuster store.

For more than a month, the five of them became regular customers. But unlike others who used that branch, they returned after dark — to watch and wait.

Then, one night last April, four of the men climbed out of a white van in matching Dickies jeans and long-sleeved shirts. Even their shoes matched: size 101/2 wide DieHard work boots, although one of the men was much smaller than the rest.

Carrying a 6-foot ladder, roofing materials, a drill and other heavy equipment, they climbed onto the Diamond Bar bank's roof. For three hours, they cut and sliced through the rooftop. Using a sonar device, they were able to locate the vault and cut a hatch right over it.

They stole nothing that night — in fact, they sealed the hole up before leaving. This was a two-night job, and they wanted to make sure bank employees never suspected a thing.

Outside the Citibank that night, the crew's lookout sat in another car drinking a 16-ounce Rockstar energy drink, a walkie-talkie in his hand.

But he wasn't the only one watching.

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Bank robbers always have used pretty much the same modus operandi: in through the front door, armed. These thieves were different. They didn't use threats or guns — just their drilling equipment.

Since a gang broke through the reinforced roof of a Laguna Niguel bank and blew a hole in the safe nearly four decades ago, it's been extremely rare in Southern California for thieves to bust in from above to steal millions.

But this new crew was taking advantage of less-fortified one-story strip-mall banks with cuttable roofs and more easily breached after-market concrete vaults, investigators say. Although those banks don't keep a lot of their cash in the vaults, some of their customers do, particularly in the wealthy, heavily Asian suburbs of the San Gabriel and Pomona valleys.

"This type of crime is a real rarity," said William J. Rehder, a retired FBI agent and author of "Where the Money Is: True Tales from the Bank Robbery Capital of the World."

Rehder, who spent decades with the FBI's L.A. bank robbery squad, said the rooftop bank jobs require real skill. "They got some real big takes in these burglaries," he said. "They chose banks in strip malls, and the setting makes it all the more easy."

When employees at the East West Bank in Rowland Heights opened the vault on a Monday morning in August 2011, the air was filled with gray dust, and 65 safe-deposit boxes lay scattered on the floor. A million dollars in bank cash was gone. The safe-deposit boxes, which once held as much as $14 million in cash, gold and jewelry, were empty.

The thieves left only one clue: the 2-by-3 holes they had cut in the ceiling before carefully replacing the roof material so anyone seeing the bank from the air wouldn't suspect it been compromised.

"There aren't too many rooftop heist serial crews this sophisticated," said Det. Nicholas Cannis of the L.A. County Sheriff's Major Crimes Bureau. "We knew this wasn't first time they cut into a building to do a burglary."

The Preferred Bank a few miles away in a Diamond Bar strip mall was hit four months later. Though the thieves never got into the vault, the M.O. was identical. Detectives believe they found the vault too difficult to breach and abandoned the job.

Then, on Sept. 8, 2012, thieves broke into the vault of the BBCN Bank on Diamond Bar Boulevard. Once inside, they stole $430,000 in bank cash and pried open 60 safety deposit boxes for $2 million more in cash, jewels and property, insurance claims indicate.

Cannis and his partner, Det. Randall Rue-Las, joined the hunt and began to scour the casebooks for thieves who had sliced through Southern California rooftops to the businesses below.

And this time, the bank burglars had left a clue: the plastic back of a walkie-talkie amid the dust. Sheriff's forensic specialists lifted some DNA off the surface and entered it in the state database.