At least one shot probably came from the curb-side sewer located on Elm Street at the bottom of the grassy knoll. The picture on the left was taken from the exact spot where JFK was sitting in the limo at Zapruder Frame 313.
These next pictures were taken just outside the sewer looking towards the approaching limo.
The curb-side sewer on Elm Street was the ideal location for a second gunman. He could not be seen entering the sewer system from a remote location, taking a shot at the President, then exiting Dealey Plaza underground without ever being observed nor photographed.
It may be true that the shooter in the sewer might not have been able to see the limo with his naked eye, and that a normal size rifle could be too large to maneuver inside of the sewer. However, a modified weapon was most likely used along with a scope (or even a periscope) which extended a couple of inches out of the sewer. Such devices existed long before 1963, and could have easily been designed or adjusted to fit the specific circumstances. The assassination was obviously planned and rehearsed in advance.
JFK Home Page