NEW YORK -- A number of high-profile brokers have left Bank of America Corp's Merrill Lynch wealth management unit in recent weeks, and top executives at the company have grown concerned enough to ask business head John Thiel to explain the departures, three sources familiar with the matter told Reuters.

Among those who have exited are top-producing brokers like Brian and Tim Brice, who oversaw around $4.5 billion in client assets in a suburb of Detroit, and whose father had also worked for Merrill. The brothers had been with Merrill Lynch for decades, before joining Morgan Stanley in September.

Raymond George, a 23-year veteran of Merrill Lynch, has also left. George joined Morgan Stanley's office in Garden City, New York in October. Eugene Montoya, who had been at Merrill for more than 40 years, decamped in September for Wells Fargo Advisors' office in Miami.

The Brice brothers declined to comment through a Morgan Stanley spokesman. George did not return a call seeking comment, and a spokeswoman for Morgan Stanley declined to comment on his departure. Montoya did not return a call and an email seeking comment.

Bank of America Chief Executive Brian Moynihan and Vice Chairman David Darnell have both asked Thiel to account for the trend of recent departures, the sources said.

In an e-mailed statement, Darnell told Reuters, "John (Thiel) is doing exactly what Brian (Moynihan) and I need him to do." Thiel did not return two calls seeking comment.

David Walker, a spokesman for Merrill Lynch's wealth management business, told Reuters in an e-mailed statement, "The facts are that advisor attrition is at historic lows and our business has never been stronger. Our results, by every measure, show we're headed in the right direction."

The reasons for the departures of the brokers, who represent a relatively small percentage of the bank's 14,000-strong force, are unclear.

While Merrill said it managed to gain 155 brokers on a net basis in the third quarter, sources said that in recent weeks the bank has been losing more brokers than it gained. In one week in October, 13 left and just one joined, one of the sources said.

Two of the sources familiar with the matter added that many of the brokers who departed had spent the majority of their careers at Merrill, while two-thirds of the additions in the third quarter were relatively inexperienced graduates of Merrill's training program. Wealth management is one of the few areas where Wall Street banks are looking to expand or at least to hang onto staff, because the business produces relatively stable returns.