By Kirsten Grind And Gregory Zuckerman

After Bill Gross abruptly quit Pacific Investment Management Co., the bond-fund giant he co-founded, in September to join a much smaller rival, the big question was how much money would follow him.

His new firm, Janus Capital Group Inc., subsequently disclosed that investors poured about $1.1 billion into Mr. Gross's new fund in October and November. That money was critical, because it helped push the Janus Global Unconstrained Bond fund past $1 billion in assets under management, a key threshold for large investors, according to industry experts. Despite Mr. Gross's status as a star fund manager, some large pension funds and their consultants have said they couldn't consider a fund with less than $1 billion in assets.

But what Janus didn't tell investors was that, in a previously unreported development, a majority of the money came from a single Southern California brokerage office--the same office where one of Mr. Gross's personal financial advisers works, according to industry executives who have viewed confidential brokerage data.

The Morgan Stanley wealth-management office in La Jolla, Calif., routed more than $700 million to Mr. Gross's Janus fund in October and November, according to people who viewed brokerage-firm fund-asset data from Albridge, a data firm owned by Bank of New York Mellon Corp. The transfers accounted for more than 60% of the money raised by Mr. Gross in the first few months after he left Pimco.

It isn't clear whether the money originated from one investor or one financial adviser, or from more than one, although Mr. Gross acknowledges at least some of the money came from him. While it isn't unusual for a manager to invest in his own fund, industry executives say it is extremely rare for one firm or office to account for such a large percentage of a prominent fund's incoming cash.

A spokesman for Denver-based Janus said the firm doesn't comment on specific fund shareholders and their investments "as a matter of policy and out of respect for the privacy of the firms we serve and their mutual fund investors." The spokesman also wouldn't comment on how much of the fund is made up of Mr. Gross's money. A spokeswoman for Morgan Stanley declined to comment. A spokesman for Albridge said, "Any data generated from Albridge Analytics' system is confidential and public use of the data is neither authorized nor permitted."

After questions from The Wall Street Journal, Janus Capital on Wednesday tweeted a comment attributed to Mr. Gross:

Janus declined to comment further.

Mr. Gross, 70 years old, is a billionaire who earned more than $200 million a year before he left Pimco, the giant money manager he helped start four decades ago and then abruptly left in September. His departure shocked the investment community and triggered a scrum for tens of billions of dollars, as rivals tried to poach Pimco's clients. Investors pulled $150 billion from Pimco last year, most of it in the months after Mr. Gross's departure. In the latest example, New York City's five pension funds withdrew nearly $5 billion from Pimco, a spokesman for New York City Comptroller Scott Stringer said Wednesday.

Investors in Janus shares have bet his arrival would help the beleaguered money manager--which has suffered from years of outflows due to poor performance--attract investors. Janus's stock jumped 43% on Sept. 26, 2014, the day Mr. Gross's hire was announced, the biggest one-day gain in the company's history. Janus shares have pulled back since then but are still up 11% since he took over the fund in early October, and the firm saw more inflows in October across all of its mutual funds than it has in any single month since 2007, according to data tracker Morningstar Inc.

"Obviously, he's off to a great start raising a lot of attention and assets," Dick Weil, Janus's chief executive, recently said of Mr. Gross.

The La Jolla office of Morgan Stanley is in a small, affluent community just north of San Diego and about 80 miles south of Mr. Gross's Janus office in Newport Beach. The La Jolla office is where one of Mr. Gross's own financial advisers, Robert Inbody, works, according to people familiar with his role. Mr. Inbody referred all queries to Morgan Stanley.

Mr. Gross doesn't live in La Jolla, but it isn't unusual for wealthy individuals to have multiple financial advisers in different locations, say brokers.

Albridge provides data from the four largest financial firms selling mutual funds to investors: Bank of America Merrill Lynch, UBS AG, Wells Fargo & Co. and Morgan Stanley. The data, covering the majority of mutual-fund sales, is provided to clients who pay a fee to subscribe.

In October, according to the Albridge data viewed by industry executives, more than $300 million of total investor inflows into Mr. Gross's Janus fund came from the Morgan Stanley office in La Jolla. According to Morningstar, the fund saw a total of about $360 million of inflows that month, meaning 87% of all new money that came into the fund in Mr. Gross's first full month on the job originated from that office. In November, the office contributed about $400 million out of $770 million in total inflows, according to the people who viewed the Albridge data and Morningstar.

When Mr. Gross took over the fund at Janus in early October, it had just $12 million in assets under management following its launch in May 2014, according to Morningstar. The firm was expected to report new asset totals for December on Wednesday. The fund follows a different strategy than Mr. Gross adhered to while running the Total Return fund at Pimco, allowing him to invest in a variety of bonds and other securities. The fund has lost 1.1% since Mr. Gross took over Oct. 6 through Tuesday, ranking it in the 52nd percentile in its category, according to Morningstar.

Janus rivals such as Vanguard Group and BlackRock Inc. appear to have taken a much bigger share of the outflows from Pimco's mutual funds, which totaled about $150 billion in 2014, according to Morningstar's most recent data. Vanguard, for example, saw $10.3 billion flow into its Total Bond Market Index fund in October, the fund's highest monthly inflow on record.

Mr. Gross got a boost in November when George Soros's Soros Fund Management made a $500 million investment, although that amount is run through a separate account at Janus that isn't part of Mr. Gross's mutual fund.

Fund managers are required by a Securities and Exchange Commission regulation to report the amount, within a range, that they invest in mutual funds that they manage. Morningstar shows no investment from Mr. Gross in the fund, though it wasn't clear whether that was due to a potential lag in reporting schedules. Janus declined to comment on its regulatory reporting.

When Mr. Gross ran Pimco's flagship Total Return fund, he regularly invested his own money, according to people familiar with the matter. Before he left Pimco, Mr. Gross held an investment of $1 million or more in the Total Return fund, according to Morningstar.

Mr. Inbody of Morgan Stanley is one of the firm's biggest producers in Southern California, according to people familiar with the matter. His LinkedIn profile says he has been a managing director at the firm since 1971.

Write to Kirsten Grind at kirsten.grind@wsj.com and Gregory Zuckerman at gregory.zuckerman@wsj.com

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