He can’t. If you recall he refused himself.
I don’t know the answer to this, so just asking. Sessions recused himself from the Russia investigation. Nunes said what he found had absolutely nothing to do with Russia. If that’s the case, then can’t Sessions involve himself in these latest revelations? Sessions is Comey’s boss and Nunes said the NSA had provided what he asked for but that the FBI had not - seems to me that Sessions could and should order that the documentation, etc. in Nunes’ letter be complied with immediately.
As an aside, I’m concerned about McCabe still being #2 in the FBI. As you will recall, his wife got a lot of $$$s from McAwful (and IIRC Clinton) for her run as a state senator in VA. Thankfully, she lost.