That is the way it works in theory. But at Citigroup, many say, it was a bit different.

David C. Bushnell was the senior risk officer who, with help from his staff, was supposed to keep an eye on the bank’s bond trading business and its multibillion-dollar portfolio of mortgage-backed securities. Those activities were part of what the bank called its fixed-income business, which Mr. Maheras supervised.

One of Mr. Maheras’s trusted deputies, Randolph H. Barker, helped oversee the huge build-up in mortgage-related securities at Citigroup. But Mr. Bushnell, Mr. Maheras and Mr. Barker were all old friends, having climbed the bank’s corporate ladder together.

It was common in the bank to see Mr. Bushnell waiting patiently  sometimes as long as 45 minutes  outside Mr. Barker’s office so he could drive him home to Short Hills, N.J., where both of their families lived. The two men took occasional fly-fishing trips together; one expedition left them stuck on a lake after their boat ran out of gas.

Because Mr. Bushnell had to monitor traders working for Mr. Barker’s bond desk, their friendship raised eyebrows inside the company among those concerned about its controls.

After all, traders’ livelihoods depended on finding new ways to make money, sometimes using methods that might not be in the bank’s long-term interests. But insufficient boundaries were established in the bank’s fixed-income unit to limit potential conflicts of interest involving Mr. Bushnell and Mr. Barker, people inside the bank say.

Indeed, some at Citigroup say that if traders or bankers wanted to complete a potentially profitable deal, they could sometimes rely on Mr. Barker to convince Mr. Bushnell that it was a risk worth taking.

Risk management “has to be independent, and it wasn’t independent at Citigroup, at least when it came to fixed income,” said one former executive in Mr. Barker’s group who, like many other people interviewed for this article, insisted on anonymity because of pending litigation against the bank or to retain close ties to their colleagues. “We used to say that if we wanted to get a deal done, we needed to convince Randy first because he could get it through.”