From an office tower overlooking the sumptuous Fashion Island mall in Newport Beach, Mohamed El-Erian sat atop a massive treasure trove of wealth as chief executive of Pimco.

Soft-spoken and cerebral, El-Erian presided over a financial juggernaut that rose to become one of the biggest and most influential investment firms in the world. His ubiquitous appearances on cable news shows could sway financial markets in a split second.

On Tuesday, El-Erian unexpectedly announced his resignation as CEO.

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Pimco offered no explanation for the departure, sparking conjecture on Wall Street about whether the current challenges at the investment giant played a role.

"He was doing CNBC and Bloomberg and all the editorials to make him so prominent — and then, poof," said Marilyn Cohen, president of Envision Capital Management in Los Angeles. "To see him leave is so shocking. There was no prelude to this."

Pacific Investment Management Co. rode the decades-long surge in bonds to become a Wall Street colossus. The company manages $2 trillion in assets, including the savings of millions of individual investors in 401(k) retirement plans.

But the once-sparkling fortunes have dimmed a bit.

The outlook for its signature bond investing is threatened by changes in the financial markets, which are expected to weigh heavily on fixed-income investment returns in coming years. Investors fled bonds for stocks in 2013, in part because of Federal Reserve policies that made the equities market attractive.

The bellwether Total Return Fund, the world's largest bond fund, sustained a 2.3% loss last year, prompting disheartened investors to withdraw more than $40 billion.

Pimco has worked furiously in recent years to branch into stock investing, but with limited success.

The company said El-Erian, 55, will step down in mid-March as part of a broader management overhaul but will remain an advisor to Pimco's parent company. Douglas Hodge, chief operating officer, was named to succeed El-Erian.

The announcement was quite an about-face considering that El-Erian was widely viewed as the heir apparent to investing guru Bill Gross, Pimco's co-founder, who runs the Total Return Fund. The company stressed that Gross isn't going anywhere.

"Pimco's fully engaged," Gross, 69, wrote in an energetic tweet Tuesday that contrasted with the company's normally staid missives. "Batteries 110% charged. I'm ready to go for another 40 years!"

Gross and El-Erian did not return emails seeking additional comment.

According to published reports, El-Erian said in a letter to the staff that he has no immediate plans.

"What happens longer-term is an open question," El-Erian reportedly wrote. "I have no plans as of now. What I do know is that I am looking forward to something different."

Still, Wall Street was consumed Tuesday with competing narratives about what lay behind El-Erian's exit.

Some analysts speculated that Pimco's parent company was dissatisfied with the middling push into stock investing. Some thought El-Erian might be planning a move into national politics, perhaps as a high-level advisor to Hillary Clinton. Others said he might simply want to step back from the breakneck pace he set at Pimco.

"We may never know the back story," said Geoff Bobroff, head of a Rhode Island mutual fund consultant and advisory firm. "Pimco has suffered the last 12 months with declines in fixed income. But other managers have as well, so I can't imagine that as the reason for him to go somewhere else."

El-Erian, a Cambridge University-trained economist who once ran Harvard University's endowment, became Pimco's co-CEO in late 2007 and later took sole possession of the job. He had previously run the firm's emerging-market bond fund before departing for the prestigious job at Harvard in early 2006.