Speaking after meeting his Iranian counterpart in Damascus, Mr Moualem said the Syrian security services had devoted huge resources to their investigation into "this despicable crime" and would soon unmask the killer.
Mr Moualem declined to say whether the security forces had detained anyone in connection with the attack or name the chief suspects.
Nevertheless, Mr Moualem asserted Mughniyeh's death had "assassinated all efforts for peace" between Syria and Israel.
Nasrallah blamed Israel for Imad Mughniyeh's assassination
Analysts say his death is a significant blow to Hezbollah, which battled Israel in the 2006 Lebanon war with help from its Iranian and Syrian backers.
Hezbollah was founded in 1982 by a group of Shia Muslim clerics after the Israeli invasion of Lebanon. It has emerged in recent years as a major political and military force in Lebanon, after military successes against Israel.