I'm guessing this footage is taken about 10 minutes after the assassination.
Sawyer gives the order to lock down the building around 12:37 PM
Alyea reports getting in the building just as it is locked down.
The footage above is taken on the first floor and must be some of the earliest footage Alyea took.
Also, there's this from Truly:
Mr. BELIN. All right.
And then you got down eventually to the first floor?
Mr. TRULY. That is right.
Mr. BELIN. About how long after these shots do you think it took you to go all the way up and look around the roof and come all the way down again?
Mr. TRULY. Oh, we might have been gone between 5 and 10 minutes. It is hard to say.
Mr. BELIN. What did you do when you got back to the first floor, or what did you see?
Mr. TRULY. When I got back to the first floor, at first I didn't see anything except officers running around, reporters in the place. There was a regular madhouse.
Mr. BELIN. Had they sealed off the building yet, do you know?
Mr. TRULY. I am sure they had.
The reporter he is referring to must surely be Alyea. Truly seems to be describing what we can see in Alyea's footage.
The important point is that both men confirm the TSBD had been locked down at this point so there should only be law enforcement officials, a couple of reporters and TSBD employees in the building.
The man in question doesn't appear to be wearing a suit and tie.
I think that rules out law enforcement, reporters and management of the TSBD.
This leaves the "manual" workers - a white, male manual worker, perhaps.