Three years ago, Security Bank Corp. made a bold decision to expand from its central Georgia roots, where business was stable but slow-growing, into Atlanta’s booming suburbs.

The Macon-based bank holding company purchased three Atlanta-area banks between May 2005 and July 2006, establishing beachheads in Alpharetta, Suwanee and Woodstock.

The goal of the acquisitions, which totaled $115 million: to tap into metro Atlanta’s lucrative market for real estate construction loans.

Unfortunately for Security, the investment came at the height of the real estate boom.

After the housing bubble burst in 2007 with the subprime mortgage crisis, loans began to sour as home builders and developers struggled to make payments.

It has caused big problems for Security, Georgia’s fifth-biggest bank holding company based on assets.

In the first half of this year alone, the company wrote off $54.5 million in bad loans, compared with $23 million for all of 2007 and $2.4 million in 2006, according to public filings.

And more troubled loans are on the company’s books.

At the end of June, Security reported $177 million in loans at least 90 days past due, compared with $35 million at that same point last year.

One of the company’s six community banks, Security Bank of Gwinnett County, has seen particularly hard times. For the first half of the year, the Suwanee-based bank reported more than $80 million in loans 90 days or more past due.

The numbers were bad enough to land the Gwinnett bank atop a list of troubled Georgia banks based on an analysis called the Texas ratio.

Officials at Security Bank Corp. acknowledged the real estate downturn has caused problems at the Gwinnett bank, but they say the Texas ratio overstates the magnitude.

Lorraine D. Miller, a senior vice president with Security Bank Corp., said the numbers don’t reflect the fact that Security Bank of Gwinnett is part of a holding company, which is able to shift capital to the Gwinnett bank.

“It’s not fair to look at [Security Bank of Gwinnett] on a stand-alone basis because it’s not a stand-alone bank,” she said.

The holding company has more than $38 million in “excess capital” at its disposal, Miller said.

Miller said the bank remains well capitalized and has taken steps to control expenses and get problem loans under control.

The bank has raised $68 million in capital this year, tightened lending standards and is moving to diversify its loan portfolio. Two weeks ago, the bank announced that CEO H. Averett “Rhett” Walker had resigned.

Chris Marinac, an Atlanta-based banking analyst, said Security Bank needs to write off more of its problem loans.

“The pricing is going to be painful … but the sooner they deal with it, the better off they will be,” he said. “We’d prefer them to sell assets and stop waiting for the market to turn. Hope is not an investment strategy.”

Miller said the company doesn’t regret buying the suburban Atlanta banks. The region’s real estate market is sure to turn around, Miller said.

“It certainly would have been wonderful to have 20/20 hindsight and to have been able to make the acquisitions at more advantageous prices,” she said. “But you don’t get that chance, do you?”