Should I use "was" or "were" here?

Example: The cause of the 2008 financial crisis () the credit default swaps.

“Credit Default Swaps” includes more than one item but is a collective term for all financial instruments of a certain type.

So, should “was” be in the () or should “were” be there? It’ll make sense to use were since there were more than one credit default swap but was could be a possibility as I am describing the credit default swaps collectively as if they were one thing.