Looks like we will findout soon enough. The talk is it just a matter of time for the release of the memo. I would think most of the leg work to get these people would be sifting through log files.
Normally it is the creating and classifying organization that has the responsibility and sole authority for declassification. However, if the document impacts the security of multiple organizations, it is common practice for each such organization to vet the document for content that might uniquely impact their security -- and to have signoff on the declassification.
In such multi-organization involvement, it is not unheard-of for a higher office -- to which all affected organizations report -- to step in and have the final say on declassification. In this case that would be the White House / Presidency...
It would not surprise me for the WH, in this instance, to take the role of the declassifying and releasing authority. And I would love for that release to be done by President Trump, himself -- during Tuesday's SOTU address! '-)