There were high-fives all around Merrill Lynch headquarters in Lower Manhattan as 2006 drew to a close. The firm’s performance was breathtaking; revenue and earnings had soared, and its shares were up 40 percent for the year.

Merrill’s decision to invest heavily in the mortgage industry was paying off handsomely. So handsomely, in fact, that on Dec. 30 that year, it essentially doubled down by paying $1.3 billion for First Franklin, a lender specializing in risky mortgages.

The deal would provide Merrill with even more loans for one of its lucrative assembly lines, an operation that bundled and repackaged mortgages so they could be resold to other investors.

It was a moment to savor for E. Stanley O’Neal, Merrill’s autocratic leader, and a group of trusted lieutenants who had helped orchestrate the firm’s profitable but belated mortgage push. Two indispensable members of O’Neal’s clique were Osman Semerci, who, among other things, ran Merrill’s bond unit, and Ahmass L. Fakahany, the firm’s vice chairman and chief administrative officer.

A native of Turkey who began his career trading stocks in Istanbul, Semerci, 41, oversaw Merrill’s mortgage operation. He often played the role of tough guy, former executives say, silencing critics who warned about the risks the firm was taking.