Seems like Comey is not denying that the tapping was done, but that perhaps that Obama was behind it. I've always viewed Obama as too lazy to do anything this intricate.
“... Yet, I don’t get why Trump would tweet this...”
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Trump tweeted about this because of the noise about Sessions recusal...
What have you heard about that in the past two days???
This wiretapping has taken over the news and Trump is sitting back smiling....
“Is the new fact that has come to light that he was wiretapped?”
That’s exactly it. His tweet starts out saying “just found out...” Also, he’s been saying all along that the obama’s were nice to him in the transition and hasn’t criticized 0 much. That tweet makes a strong, direct accusation against 0, even going as far as calling him a “sick guy” evidence found at trump tower may hold the key.