Top military commander Mustafa Badreddine, who died in an explosion in Damascus, is the latest of a number of senior figures from the Lebanese Shiite terror group to be killed in past years.
In the past, Israel was the prime suspect in any assassinations, but since Hezbollah joined Syria's civil war in 2012 to support President Bashar Assad, it has lost several prominent members in combat and has gained a broader range of enemies. More than 1,000 of its foot soldiers have been killed in the Syria war, which began in 2011, compared to the 1,276 fighters killed during its 18-year guerrilla war with Israeli forces that occupied southern Lebanon until 2000.
Here's a look at possible culprits in Badreddine's killing as well as a glance of previous senior members killed.
Israel: Enmity runs deep between Israel and Hezbollah. The militant group touts itself as Lebanon's defender against Israel after the long fight against the Israeli occupation, and Israel as well as the United States considers the group a terrorist group bent on its destruction. Since the Israeli withdrawal, the two sides have come to blows several times, most disastrously in 2006, when weeks of fighting and Israeli bombardment devastated much of the south.
Experts say Israel may seize the opportunity while Hezbollah is deeply involved in Syria to take out senior group members. A Lebanese TV station close to Hezbollah initially reported an Israeli airstrike killed Badreddine but it withdrew the report. Israel has not commented.
Extremist groups: Sunni extremists linked to al-Qaida and the Islamic State group dominate the Syrian battlefield and are staunch opponents of Hezbollah. Though rivals on the ground, IS and al-Qaida's Syria branch, called the Nusra Front, consider Shiites to be apostates whose blood may be shed. Both have claimed responsibility for most of the suicide bombings in the war and attacks in Lebanon, including one against a Hezbollah stronghold in Beirut in 2015 that killed nearly 50 people. They have killed Hezbollah officers during combat in Syria, but neither group is known to have carried out targeted assassinations against Hezbollah.
Syrian rebels: Syrian rebels have been threatening Hezbollah since the group sent fighters to Syria. They have hit its positions in Syria with rockets and car bombings and battled Hezbollah forces directly on the ground.
Saudi Arabia: The kingdom does not have a known history of sponsoring assassinations or of targeting Hezbollah directly, but tensions between it and the Lebanese Shiite group are at an all-time high. Saudi Arabia backs Syria's rebels and is a bitter opponent of Iran, Hezbollah's patron. Saudi Arabia suspended a $3 billion program funding Lebanon's security forces because of Hezbollah's power there and it backed an Arab League decision to label the group a terrorist organization. Saudi Arabia has also taken a more aggressive stance around the region, fighting Shiite rebels in Yemen that it says are a proxy for Iran and calling for the creation of a Muslim military force.
Hezbollah leader Sayyed Hassan Nasrallah regularly lashes out at Saudi Arabia in his public speeches, recently likening it to Israel, saying both are fueling Sunni-Shiite hatreds in the region. "There is a history, there is a tension," said Matthew Levitt, a Washington Institute expert of Hezbollah, though he underlined that any talk of a culprit in Badreddine's death is speculation at the point.
Past senior Hezbollah figures killed
2016: Mustafa Badreddine, Hezbollah's top military commander, was killed in a blast in the Syrian capital of Damascus. Hezbollah, which announced his death on May 13, says it is investigating whether he was killed in an airstrike, missile, or artillery shelling.
Ali Fayyadh, better known as Abu Alaa Bosna, who led some of Hezbollah's military operations in Lebanon, Syria, Iraq and Bosnia was killed in February during battles with the Islamic State group near the town of Khanaser in the northern Syrian province of Aleppo.
2015: Samir Kantar, who spent 30 years in an Israeli prison for the 1979 killing of an Israel man his daughter and a policeman, was killed in December along with eight others in a suspected Israeli airstrike on a residential building in Jaramana, a Damascus suburb.
Hassan Hussein al-Haj, a top Hezbollah commander, was killed in October while fighting al-Qaida-linked fighters in the northwestern province of Idlib. A week later, his replacement, Mahdi Hassan Obeid, was killed during fighting in the same province.
An Israeli airstrike in southern Syria killed Jihad Mughniyeh and five other Hezbollah fighters in January. Mughniyeh, the son of slain Hezbollah military chief Imad Mughniyeh, was apparently in charge of operations in the Golan Heights.
2014: A senior military officer, Fawzi Ayoub, a dual Lebanese-Canadian citizen who was wanted by the FBI on charges of trying to use a forged U.S. passport to enter Israel, was killed in May in Syria's northern province of Aleppo, reportedly in an ambush by Western-backed rebels.
2013: Senior Hezbollah commander Hassan al-Laqis was assassinated by gunmen in southern Beirut in December. Hezbollah blamed Israel for the killing.
2008: Imad Mughniyeh, Hezbollah's military chief, was killed when a bomb planted in his car exploded in Damascus. Mughniyeh was one of the world's most elusive militants, accused of engineering suicide bombings during Lebanon's civil war and of planning the 1985 hijacking of a TWA airliner in which a U.S. Navy diver was killed. He had vanished from public view for 15 years before his death but had become one of the most powerful figures within Hezbollah during that time. Hezbollah blamed his killing on Israel. Mughniyeh was the brother-in-law and mentor of Badreddine, who replaced him after his death.
1992: Sheikh Abbas al-Musawi, one of the founders of Hezbollah, was killed along with his 5-year-old son and four others in an airstrike by Israeli helicopters on his motorcade in southern Lebanon.
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