The overnight killing of Hassan al-Laqis, described as a founding member of the group and one of its top commanders, was a huge blow to the Iranian-backed group that dominates power in Lebanon.
Hezbollah's heavy-handed and very open involvement in the civil war next door has enraged the overwhelmingly Sunni rebels seeking to oust Assad, and those sectarian divisions have spilled over into Lebanon and exposed the group and its Shiite supporters to retaliatory attacks.
Hezbollah strongholds have been the target of deadly car bomb attacks and suicide bombers attacked the Iranian Embassy in Beirut last month, killing 23 people.
The militant group quickly blamed its main enemy Israel for the assassination. Israeli officials denied any involvement.
Al-Laqis' killing came shortly after Hezbollah leader Sheik Hassan Nasrallah ended a three-hour interview with a local television station, in which he accused Saudi Arabia of being behind the Iranian Embassy bombings. He indirectly blamed an alliance between Iran's rivals Israel and Saudi Arabia for trying to strike at the group.
The group announced his death Hezbollah in a statement, saying al-Laqis was killed as he returned home from work around midnight.
"The brother martyr Hassan al-Laqis spent his youth and all his life in this honorable resistance since its inception up until the last moments of his life," the statement said.
An official close to Hezbollah said al-Laqis held some of the group's most sensitive portfolios and was close to Nasrallah.
A Lebanese security official and the official close to Hezbollah said al-Laqis was shot with a pistol equipped with a silencer at close range after he parked his car in the ground floor garage of his apartment building in the Hadath neighborhood, just southwest of Beirut.
He was struck by five bullets in the head and neck, the Lebanese official said, speaking on condition of anonymity in line with regulations. Al-Laqis was rushed to a nearby hospital but died early Wednesday from his wounds, officials said.
The parking lot was stained with muddied footprints that led to a small olive grove nearby. Yellow police tape blocked off the area and Hezbollah investigators were at the scene.
"I was trying to sleep, and I heard ... a bullet being fired and a dog barking," said Abdullah, a local resident who wished to be identified only with his first name for security reasons. "I did not bother myself, but later I heard people screaming. I had a look and found it was crowded, and then our neighbors told us that one of the neighbors was assassinated," Abdullah said.
The killing and other attacks underscored how the Shiite militia has found itself mired into fronts: Shoring up Assad's rule in Syria, and against the Jewish state. Hezbollah's fight in Syria marked a strategic shift for the fiercely anti-Israel group, one that some of its most loyal supporters in the Shiite community may be reluctant to embrace.
It has emboldened the group's critics in the Arab world and its Western-backed political opponents in Lebanon who blame it for dragging Lebanon into Syria's war,
The Lebanese state news agency later published a photograph it identified as al-Laqis. The image showed a man who appeared to be in his mid-40s, with neatly cut black hair and a graying close-cropped beard, wearing beige-and-khaki military clothing.
The Hezbollah statement claimed that Israel tried to kill him several times, but had failed.
"The Israeli enemy is naturally directly to blame," the statement said. "This enemy must shoulder complete responsibility and repercussions for this heinous crime and its repeated targeting of leaders and cadres of the resistance."
Israeli foreign ministry spokesman Yigal Palmor denied any involvement.
"Israel has nothing to do with this incident," Palmor said. "These automatic accusations are an innate reflex with Hezbollah. They don't need evidence, they don't need facts, they just blame anything on Israel."
Hezbollah has fought several wars against Israel. Al-Laqis' son died fighting Israel in the monthlong 2006 war. Israel's spy service has been suspected of assassinating Hezbollah commanders for more than two decades.
In 1992, Israeli helicopter gunships ambushed the motorcade of Hezbollah leader Sheik Abbas Musawi, killing him, his wife, 5-year-old son and four bodyguards. Eight years earlier, Hezbollah leader Sheik Ragheb Harb was gunned down in south Lebanon.
But one of the biggest blows for the group came in 2008 when Imad Mughniyeh, a top Hezbollah military commander, was killed by a bomb that ripped through his car in Damascus.