Japanese phrase
実は、___が原因でした。 jitsu wa ___ ga gen in de shi ta
Meaning
It turns out that ___ was the cause.
When to use it
States that the real cause has been identified. It often corrects an earlier assumption and gently adds an important fact.
Example dialogue
どうして遅れたんですか？ dō shi te oku re ta n de su ka
Why were you late?
実は、渋滞が原因でした。 jitsu wa jū tai ga gen in de shi ta
Actually, traffic caused it.