Thousands of people have attended the funeral in Lebanon's capital, Beirut, of top Hezbollah military commander Mustafa Amine Badreddine.
He died in an explosion near Damascus airport, the Lebanon-based group said, adding it would announce "within hours" its report into the killing.
Hezbollah has sent thousands of troops to support Syria's President Assad.
In 2015, the US said that Badreddine was behind all Hezbollah's military operations in Syria since 2011.
He was also charged with leading the assassination of former Lebanese PM Rafik Hariri in Beirut in 2005.
Images from the funeral showed the coffin being carried among a mass of supporters in the southern suburbs of Beirut, some of them chanting "Death to America" and Shia slogans.
The BBC's Quentin Sommerville, in the capital, says some at the funeral blamed Israel for the killing, with one mourner saying: "Hezbollah has many spies."
Another said that without Badreddine, "Daesh [another name for so-called Islamic State] would be here".
A thousand conspiracy theories: Quentin Sommerville, BBC News, Beirut
The crowd at the funeral pointed the finger at the usual suspect. Who carried out the attack, I asked three young women in black abayas: "Israel!" they replied in unison.
But the circumstances around Mustafa Badreddine's death are unclear, and have already sparked a thousand conspiracy theories.
It appears he was the militant group's top commander in Syria. Hezbollah is already stretched thin there, more than 1,600 of its fighters have been killed, and the pictures of its fresh "martyrs" increasingly show very young, or older men, rather than fighters in their prime. The group has promised to retaliate, but that will be difficult. It is already preoccupied in Syria.
And despite a pledge to avenge the death of its previous military commander, Imad Mughniyeh, killed in Damascus in 2008, it failed to do so. Mughniyeh was Badreddine's brother-in-law, the two men are now buried side by side in the same cemetery in Beirut's southern suburbs.
