Given it's size, I don't know of anything else it could be from.
Also, (according to the discovery channel at least) when you build engines that big, small problems can become big problems rather fast. In a normal engine anything large enough that it's structural stable, won't fit through the vast majority of the openings. When engines get large enough that you can crawl inside the cylinder to inspect it, the average size of the openings between various parts of the engine allows for significantly larger parts that broke off upstream to pass on through to whatever is next.
-Terry
-----
"so listen up boy, or pornography starring your mother will be the second worst thing to happen to you today"
TF2: Spy
Also, (according to the discovery channel at least) when you build engines that big, small problems can become big problems rather fast. In a normal engine anything large enough that it's structural stable, won't fit through the vast majority of the openings. When engines get large enough that you can crawl inside the cylinder to inspect it, the average size of the openings between various parts of the engine allows for significantly larger parts that broke off upstream to pass on through to whatever is next.
-Terry
-----
"so listen up boy, or pornography starring your mother will be the second worst thing to happen to you today"
TF2: Spy