BEIRUT, Lebanon — The Lebanese militant group Hezbollah declared Saturday that Mustafa Amine Badreddine, the senior commander who died in Syria this past week, had been killed in an artillery attack by insurgents whom the group has been fighting for the past four years.

Hezbollah said in a statement that the killing would increase “our determination to continue the fight against these criminal gangs and defeat them,” blaming “takfiri groups,” a term for Islamist extremists that Hezbollah broadly applies to insurgents opposed to its ally in Syria, President Bashar al-Assad.

But the declaration raised more questions than it answered, as fierce speculation continued about how, when, where and by whom Mr. Badreddine was killed, and what the death reveals about the state of Hezbollah and its war effort in Syria.

A day earlier, when Hezbollah announced his death, Lebanese media outlets sympathetic to the group initially blamed Israel, the group’s main enemy, for the attack. Israel is widely believed to have assassinated several senior Hezbollah commanders over the past few decades and has carried out a number of airstrikes in Syria, officially unacknowledged but confirmed by Western officials, against the group.