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As the U.S. government spends an unprecedented amount of money to fix the economy, there is an equally great need to raise the cash to pay for it. This is accomplished through borrowing, whereby Uncle Sam sells Treasury securities of varying maturity.

For investors, government bills, notes and bonds are considered safe because they have a guaranteed rate of return, based on faith in future U.S. tax revenues. The government has been partially funding operations via Treasury securities for decades.

This borrowing adds to the national debt, which has recently surpassed $15 trillion and is rising every second. The amount of debt is quickly approaching the federal debt ceiling, a legal limit to borrowing that currently stands at $16.4 trillion.

Much of that debt is held by private sector, but about 40 percent is held by public entities, including parts of the government. Here's who owns the most. Foreign countries listed include private and public investors, according to monthly U.S. Treasury data.

1. Federal Reserve and Intragovernmental Holdings

U.S. debt holdings: $6.328 trillion

That’s right, the biggest single holder of U.S. government debt is the Federal Reserve system. The Fed's system of banks and other U.S. intragovernmental holdings accounted for a stunning $6.328 trillion in U.S. Treasury debt in Spetember 2011 (the most recent number available). The amount is an all-time high as the Federal Reserve continues to expand its balance sheet, partially to purchase U.S. government debt securities.

About a decade ago, the total government holdings were "only" $2.5 trillion.

2. China


Photo: DAJ RM | Getty Images
U.S. debt holdings: $1.132 trillion

The largest foreign holder of U.S. Treasury securities, China currently has $1.132 trillion in American debt, although it is down from all time highs of $1.173 trillion in July 2011. For more on China and currency, see CNBC Explains.



3. Other Investors/Savings Bonds

U.S. debt holdings $1.107 trillion

With the most recent numbers from June 2011, this extremely diverse group includes individuals, government-sponsored enterprises, brokers and dealers, bank personal trusts, estates, savings bonds, corporate and noncorporate businesses for a total of $1.107 trillion.

Although the level of debt held in U.S. savings bonds has remained basically constant since 2000, the broad category of "other" investors has nearly quadrupled since reaching a four-year low in December 2007.

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4. Japan


Photo: AP
U.S. debt holdings: $1.038 trillion

One of the U.S.'s largest trade partners, Japan is also one of the U.S.'s largest debt holders, currently owning $1.038 trillion in Treasury securities.




5. Pension Funds

U.S. debt holdings: $842.2 billion

Pension funds control large amounts of money, reserved for personal retirements, and thus are obligated to make relatively safe investments. This group, which includes private and local government pension funds, holds $842.2 billion in U.S. debt. The private pension fund category also includes U.S. Treasury securities held by the Federal Employees Retirement System Thrift Savings Plan G Fund.

6. Mutual Funds

U.S. debt holdings: $653.5 billion

According to the Federal Reserve, mutual funds hold the sixth-largest amount of U.S. debt compared to any other group, although mutual fund holdings have diminished by more than $105 billion since December 2008. Including money market funds, mutual funds and closed-end funds, this group of investments managed about $653.5 billion in U.S. Treasury securities as of June 2011, which are the most recent numbers available.

7. State and Local Governments

U.S. debt holdings: $484.4 billion

U.S. state and local governments have nearly a half-trillion dollars invested in American debt, according to the Federal Reserve. The level of investment has remained stable since 2006, moving within the range of $484 billion and $576 billion. The current debt holdings, however, represent the lowest aggregate level for state and local governments since December 2005, when they stood at $481.4 billion.

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8. The United Kingdom


Photo: Dominic Burke | Getty Images
U.S. debt holdings: $429.4 billion

The U.K. currently holds $429.4 billion in U.S. debt, but the country's investment has fluctuated dramatically during the past two years. Now at its all-time high (and rapidly increasing), British holdings were as low as $55 billion in June 2008.




9. Depository Institutions

U.S. debt holdings: $284.5 billion

As of June 2011 (the most recent numbers available), the Federal Reserve Board of Governors lists depository institutions as holding about $284.5 billion in U.S. debt.

This group includes commercial banks, savings banks and credit unions. In 2011, its holdings more than tripled from the 2008 low of $105 billion. Between June and September 2011, holdings for depository institutions fell by nearly $44 billion.

10. Insurance Companies


Photo: Sylvain Leprovost
U.S. debt holdings: $250.1 billion

According to the Federal Reserve Board of Governors, insurance companies hold $250.1 billion in Treasury securities. This group includes property-casualty and life insurance firms.

Biggest Holders of US Government Debt - Yahoo! Finance

SmashMouth

Somehow.... it does not add up to 15 Trillion plus Dollars.