“Banks continue to buy Treasuries as we move further through the economic cycle,” said Jeff Caughron, chief operating officer at Oklahoma City-based Baker Group, which advises community banks with over $45 billion in investments. And there are just “fewer good loans out there to be made.”

That may help contain the backup in Treasury yields as rates rise and foreign demand slows.

Yields on the U.S. 10-year note have climbed about a half-percentage point since falling to an all-time low of 1.318 percent in July. They were 1.84 percent Monday morning in London. Losses have accelerated as bonds globally head for their worst month in about six years on speculation major central banks are moving closer to reining in stimulus.

But with banks finding fewer opportunities to lend, it's raising deeper questions about the prospects for future economic growth.

Commercial banks in the U.S. have amassed $90 billion of Treasuries and non-mortgage debt from federal agencies this year alone, bringing the total to $754 billion, according to data compiled by the Fed. Including federally guaranteed mortgage-backed securities, banks now own $2.4 trillion of government bonds, which would be the most since the central bank began compiling data in 1973.

The five biggest lenders — Wells Fargo & Co., JPMorgan Chase & Co., Bank of America Corp., Citigroup Inc. and U.S. Bancorp — held a combined $206 billion of government debt at the end of the second quarter, according to the latest available filings. That's a 74 percent increase over the past three years.

Banks aren't lending more because the economy “isn't growing as fast as we'd like it to grow,” said Paul Miller, a bank analyst at FBR Capital Markets & Co. “Banks are only able to take a certain amount of risk.”

Bank of America, the only big lender to provide supplemental data on its investments through September, has doubled its “available-for-sale” holdings in government debt to $45 billion over the past six months. AFS excludes securities used in daily trading and those intended to be held until maturity. Citigroup has also increased its exposure in a big way, boosting its holdings to $126 billion at the end of June, the most since at least 1999.

What's more, banking organizations across America have boosted the stakes they plan to hold to maturity (meaning they can keep those investments on their books at the price they paid and not worry about market fluctuations) to almost a half-trillion dollars in the second quarter, based on New York Fed data. At the end of 2013, the figure was just $50 billion.

A big reason banks are funneling so much money into safe assets is that deposit growth is outstripping loan demand. For the biggest lenders, deposits increased by 6.7 percent to $5.2 trillion in the third quarter from a year earlier, exceeding loan growth by a full percentage point.

Few Alternatives

“We have more deposits coming in than we have loans,” Brian Moynihan, Bank of America's CEO, said on the bank's Oct. 17 third-quarter earnings call. “We don't take credit risk there, and so we have two alternatives, Treasuries and mortgage-backed securities.”

Over the past year, the rate of Bank of America's commercial-loan growth has fallen from more than 10 percent to less than 5 percent. At the same time, consumer lending remains stagnant, contracting for a 23rd consecutive quarter. Citigroup's retail loan business has also shrunk, while JPMorgan and Wells Fargo, two of the strongest banking franchises in the U.S., have seen gains in their overall lending slow in recent quarters as well.

Instead of leaving idle cash parked at the Fed, where its earns only 0.5 percent, U.S. government bonds are providing banks an increasingly attractive spread over what they pay depositors. Two-year Treasury notes yield 0.85 percent, while the average deposit rate for interest-bearing checking accounts is 0.31 percent, according to Bankrate.com.

For JPMorgan, the gap between yields on two-year U.S. notes and the bank's average deposit rate has been roughly 0.64 percentage point this year, the widest in a decade, data compiled by Bloomberg show.

“Banks need yield wherever they can so this is an interest-margin story also,” said George Goncalves, the head of U.S. rates research at Nomura Holdings Inc.

Banks have also become more reluctant to take on too many risks and expose themselves to potential loan losses, especially as growth remains lackluster and borrowers seek to reduce their indebtedness. According to the Fed's quarterly senior loan officer survey, a majority of large and medium-sized U.S. lenders have tightened lending standards for four straight quarters. That hasn't happened since 2009, when the U.S. was still struggling to emerge from the worst economic crisis in seven decades.

Even banks that are faring better have grown more cautious.

“We are being very selective in what loans we make and who we make one to,” said Thomas Wornham, CEO and president of San Diego Private Bancorp of America, which extended loans at almost twice the rate of its deposit growth last quarter. “What you are seeing out there is a slowdown.”

Both Ways

To some, that's just how banks should behave, since excessive risk-taking was exactly what led to the financial crisis.

“It's one of those paradoxes,” said David Keeble, the head of fixed-income strategy at Credit Agricole SA. “The Fed probably wants it both ways — they want more lending to keep the economy going, but more safe lending, which means less lending.”

Whatever the case, it's clear that while the excess liquidity from the Fed's easy-money policies has been a boon for government securities, banks are still wary of putting much of the cash to work.

“Banks have lots of customer deposits and corporations aren't spending cash,” said Moorad Choudhry, a professor in the business school at University of Kent and the author of more than a dozen books on finance including “The Principles of Banking.” “There is nowhere else for banks to put this money.”

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