Shushan was shot in the head twice at point-blank range by an unknown assailant who managed to escape before police arrived the scene.
"Hundreds pass by here every day to be blessed by the rabbi; it was lucky that we noticed the suspect."
The report noted that the assassination was carried out using a "one-ton remote-controlled gun" that was smuggled into the country in pieces.
Slim was a vocal critic of what he described as armed group Hezbollah's intimidation tactics and attempts to monopolise Lebanese politics.
Another man was also injured in the shooting, and was evacuated to Wolfson Hospital independently. He is classified as being in serious condition.
Prosecutors said Miller made numerous threatening remarks online, including one instance in which he commented "next time we bring the guns"
The comments by Acting US Attorney Michael Sherwin appeared to be an effort to walk back claims that federal prosecutors in Arizona had made in a court filing late on Thursday.
Officials: Transmission picked up by air traffic control a hoax
Afghan government officials have in recent weeks accused the Taliban of a string of high-profile murders, including of bureaucrats and journalists, and bomb attacks.
A civil society activist was also killed on Friday by unknown militants.