Fugitive Mob Boss James "Whitey" Bulger Lived Like a Beachside Retiree

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Just a block from the Santa Monica Promenade, today a parade of tourists from Boston passed by the apartment building where notorious hit man James "Whitey" Bulger was arrested yesterday after 16 years on the lam.

Snapping keepsake photos for posterity and to send back home to a town where "Bulger" is a household name, one tourist called Whitey Bulger, believed to be responsible for 19 murders,"(O)ur bin Laden."

It took 16 years this time, but the FBI finally got its man. Just weeks after the U.S. took down bin Laden in a spectacular raid on his Pakistan compound, they nabbed Bulger, also on the top 10 "Most Wanted" list, in an uneventful arrest outside his third-floor apartment.

Before he went on the run, Bulger presided over organized crime in South Boston as the head of the Winter Hill Gang.

It's the FBI's moment to preen; the arrest ends an embarrassing chapter for the bureau. But many questions remain about how the brutal criminal was able to hide in plain sight for so long.

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Bulger, 81, was arrested along with his longtime companion Catherine Greig, 60, who, it appears, led to his downfall.

Earlier this week, the FBI ran TV ads – targeted to shows with heavy female viewership - in 14 media markets, on Facebook, the FBI website and other social media tools. Agents were hoping that someone had spotted Whitey's lady friend - perhaps in a hair salon, at a doctor's office -- and that she would lead them to the aging criminal. Within hours of launching the effort, the tip they'd hoped for came in.

Details have yet to emerge about how the elderly Bulger, who did not appear healthy when he was arrested, and Greig spent their days, how they paid their bills, how they managed to live under the radar for so long.

ABC News has learned from neighbor Barbara Gluck that Bulger may be suffering from dementia, perhaps triggering the bouts of rage that Gluck said she'd witnessed when Greig attempted to be social with neighbors.

Bulger and Greig, according to neighbors, may have lived in the building of one- and two-bedroom apartments, called The Princess Eugenia, for more than 10 years, living a quiet life under the assumed names of Charles and Carol Gasco.

"I just remember him being quiet and keeping to himself," another neighbor told ABC News.

Just how hard is it to maintain anonymity and evade the detection of law enforcement for the better part of two decades?

"Not that difficult,'' it turns out, said former FBI agent and ABC News Consultant Brad Garrett, an expert on crime and terrorism. "Live as low-key a life as you can, have a cover story as to who you are and why you're there," he said. And "stick to it."

A slight change in appearance and choosing to live in a place where you have the ability to blend in are essential, he added.

The FBI believes that Bulger planned his life as a fugitive for a long time before he went on the run, in January of 1995, when he was tipped off that authorities were about to arrest him.

"There's a fairly consistent story that he had safety deposit boxes in London, Ireland and elsewhere," Garrett said.

When agents and police searched Whitey Bulger's residence in Santa Monica they found approximately $800,000 in cash, and a stash of weapons including sawed-off shotguns, Mac 10 machine pistols and 15 assault rifles. The arsenal totaled about 60 weapons.

Bulger and Greig paid for their rent with cash, ABC has learned. And they likely paid for the rest of their expenses the same way, Garrett suggested, so as not leave a trail.

Garrett suggests, "(Bulger) probably had an informal pipeline, money, cash that he had access to," perhaps a few friends "who fed him money during direct handoffs." In this scenario, it is likely that the contact would meet him in a public spot and "hand him a brown bag with money in it," Garrett said.

This, he said, is how the Whiteys of the world operate. And, as such, unless authorities know who was aiding him, "You have no trail. No electronic trail, no digital trail."

Younger criminals would probably have a harder shot at success, he said. Bulger likely never grew accustomed to the digital world most people today live in. He may never have used e-mail.

Authorities say bin Laden too stayed off the grid in his hideout in Pakistan, helping him avoid capture.

It is also likely that Bulger avoided his old life as an alleged mob kingpin. "What happens is, people go back and they touch their past lives and when you touch your past life, you're going to get caught," said Garrett.

"He's been lucky and he's always been disciplined."

ABC News' David Wright and Richard Esposito contributed to this report.