It would make sense that the the authorities had some idea something was going down because at that point, they had confirmed the library bomb, and then going with the known SOP for Arab terrorists overseas, it would mean someone is set up out of range but in sight of the bombs and then would trigger them by command from a distance.

So they see a guy alone on a rooftop, start to figure stuff out, shoot him, he is bleeding out while his cohort is placing the last bomb, triggers the bombs prematurely before he loses consciousness but while his cohort had not quite gotten out of the blast radius.

Kind of makes sense to me. Fully explains why the guy in custody is mainly a "person of interest" and not solely the suspect. He might have been a patsy who got conned into placing the bomb, so the real question the Israelis are asking is about who got shot on one of the nearby rooftops and by whom.