If a witness told Sawyer that he had seen a man running away from the building with a rifle, why would Sawyer then state that it was unknown if he was still in the building and not known if he was there in the first place?
Because Insp. Sawyer understands that it is possible the man, if an employee in the building, tossed the rifle and returned to the building to blend in; but that it is also possible he didn't fire from that building at all. He is cautioning against drawing conclusions.
Hence the response to Insp. Sawyer's broadcast: "531 (Sergeant G.D. Henslee) Well, all the information we have receive, 9, indicates that it did come from about the 5th or 4th floor of that building."
i.e. everything points to the fact that this did come from the building, so we can be confident the man you describe was in that building in the first place (i.e. at the time of the shooting)
Sgt. Henslee would not need to be pointing this out to Insp. Sawyer if the latter had gotten his description from Mr Brennan.
Again, this isn't speculation. Law enforcement knew all about it: