American International Group Inc. is seeking a chief investment officer to oversee a portfolio valued at more than $350 billion, according to a person with knowledge of the insurer’s plan.

The money manager would take over duties handled by Executive Vice President William Dooley, who is nearing retirement after working for the company since 1978, said the person, who asked not to be identified because the search process is private.

AIG has been expanding its allocation to commercial mortgage-backed securities and other structured debt amid a decline in fixed-income yields. The New York-based company had about $280 billion in bonds as of March 31, with more than half in corporate debt.

“You absolutely need somebody who is very strategic, a good manager, but also has expertise in a lot of asset classes,” Josh Stirling, an analyst at Sanford C. Bernstein & Co., said in an interview. “AIGhas simplified itself dramatically but it’s still a very large complicated institution,” he said. “They still have a fair amount of exotic securities.”

Monika Machon held the CIO post until 2013, when she was named treasurer. Brian Schreiber and Geoff Cornell were given the title of deputy CIOs, working under Dooley, when Machon switched jobs. Schreiber was named in May as chief strategy officer, a role that includes acquisitions and divestitures.

Hancock’s Moves

Chief Executive Officer Peter Hancock has been shaking up management since taking over in September. He reorganized the company into two main divisions. One, led by John Doyle, focuses on commercial clients, while Kevin Hogan oversees the consumer operation. The previous split was between one unit selling property-casualty coverage, which was led by Hancock until his promotion, and another operation selling life insurance and retirement products.

Hancock has also added new positions, hiring a chief information officer and chief technology officer. Jon Diat, a spokesman for AIG, declined to comment.

Schreiber’s portion of the portfolio included holdings such as real estate, private equity and hedge funds. He helped last year to set up a commercial-lending venture that the insurer co- founded with private equity firm Oak Hill Capital Management. Cornell has overseen corporate and municipal securities.

First-quarter investment income fell 8.5 percent to $3.84 billion, AIG said in April. Insurers hold bonds to back obligations to policyholders and generate profits for shareholders.