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FBI Director James Comey says extreme situations call for extreme measures ... which is why he's actually revealing there IS an open investigation into Russia and the presidential election.

Comey testified on Capitol Hill Monday morning the Bureau is looking into whether the Russians really did interfere, and also whether anyone in Donald Trump's camp was communicating with Russia.

He said it's a break from policy to go public this way, but added there will still be some limits about what's revealed.

As for Donald Trump's claims President Obama was wiretapping him ... Comey said the Bureau has "no information" to support that.