This post examines which shooter locations are consistent with the 0.07 second and 0.017 echo delays seen in source 4. First note that there are only two suitable surfaces for the echoes, AB (cyan), and CD (magenta). To cause an early echo (~0.07s) audible at source 4, you have to be in the region denoted by magenta dots. Similarly, to cause a later echo (~0.17s) audible at source 4, you have to be among the cyan dots. Some of these regions are otherwise occluded, and do not really work, but all the useful solutions are inside these regions.
Suppose, for the sake of argument, that there are two shooters, one taking shots 1-3, the other 4-8. As recent checking of greg_n’s dataset shows, the two shooters need to be within about 10 feet of each other. Where is this possible, with each shooter generating only one primary echo? Right where Crooks was operating. You could deduce this location using only the audio from source 4 and a map.
Is there a plausible scenario for one shooter? Yes. Start Crooks back some from the crown, and assign him shots 1-3. Those are early echoes. Now Trump drops, so Crooks moves forward a few feet to get a better view. He has crossed from early echoes to later echoes. Assign him shots 4-8 too.
This diagram is sensitive to the positions of A,B,C,D, and [4], so take it as close, but not intended to be correct to the foot.
Finally, the blue line shows the locus of points from which shots will create echo delays of 0.0218 seconds [later post will reconcile with 0.017s observed (some sound travel is in steel)]. The red line shows the locus of points from which shots will create echo delays of 0.079 seconds. These lines are also sensitive to the source and reflector geometries, so should only be taken as representative of a plausible accounting.
In summary, the source 4 echoes do not demand a second shooter to explain them. Moreover, to produce the echoes seen, you have to be shooting from Crook’s approximate location, certainly not far from that rooftop.
Finally, it is clearly not possible to fire from a south-facing aperture in the vertical south wall of building 6 and get the early echoes at [4].