Of the (current) national of about $20T:
1) Foreign, including China (who holds 6% of our debt) at the top (just above Japan) - $6T
2) Banks, investors, mutual funds ($1.6T, larger than China), pensions, etc - $6T
3) Intragovernment debt (close to half of which being the SSI "trust" "fund") - $5.5T
4) Federal Reserve - $2.5T
SSI is actually the largest individual entity at $2.8T.
This article is rather misleading, as it looks only at sovereign foreign holders and the Fed, while completely ignoring the other 55%.
[Note: item #2 is going to include some foreign holders, though working from memory here the last time I looked most of that was domestic]