BEIRUT (AP) — Top Hezbollah officials, including deputy leader Sheikh Naim Kassem, attended a mourning ceremony at a hall in southern Beirut on Friday, to pay tribute to the Lebanese terror group’s commander in Syria, Mustafa Badreddine, who was killed earlier this week in an explosion in Damascus.

Explanations or accusations regarding the cause of the blast, however, were scant.

Badreddine, 55, had been the mastermind of the group’s involvement in Syria’s civil war since Hezbollah fighters joined the battle on the side of Syrian President Bashar Assad’s forces against those trying to remove him from power, according to pro-Hezbollah media. Hezbollah, along with Iran, has been one of Assad’s strongest backers.

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In its statement announcing his death, Hezbollah said “a strong explosion targeted one of our centers near the Damascus International Airport, leading to the martyrdom of brother commander Mustafa Badreddine and wounding several others.” It called Badreddine a “great jihadi leader,” who had joined “the convoy of martyrs, on top of them his comrade, close friend and predecessor Imad Mughniyeh.

Badreddine’s death is the biggest blow to the group since Mughniyeh’s 2008 assassination in a bomb attack in Damascus. After that, Badreddine, known among the group’s ranks as Zulfiqar, became Hezbollah’s top military commander and adviser to the group’s leader, Hassan Nasrallah.

Badreddine’s nom de guerre, Zulfiqar, was the name of double-headed sword of Imam Ali, the Prophet Muhammad’s cousin and son-in-law and the Shiite sect’s most sacred martyr.

“The message is that a martyred commander has joined the convoy of martyred leaders,” Hezbollah Cabinet Minister Hussein Haj Hassan told The Associated Press. “He boosts us with his martyrdom with strength, glory, will and intention to continue the fight against the Zionist enemy and the takfiris (Sunni extremists) until victory is achieved, God willing.”

In an emotional gesture of grief, Badreddine’s brother Adnan raised his hand to touch a giant poster of the killed Hezbollah operative. Badreddine’s only son, Ali, wept, as top Hezbollah official Hashim Safieddine hugged to comfort him.

A funeral was to be held Friday afternoon at a Shiite cemetery south of Beirut where Badreddine was to be laid to rest next to Mughniyeh, who was also his brother-in-law.

Badreddine’s death — a major blow to the Shiite group — was initially blamed on Israel, a claim that was apparently retracted. The Beirut-based Al-Mayadeen TV, which is close to Hezbollah, at first said he was killed in an IAF airstrike, but later removed the report.

There was little information as to how he was killed. Hezbollah said the attack occurred near the Damascus airport without giving details. The airport is close to the Shiite shrine of Sayyida Zeinab where the group has wide presence and several military positions.

Hezbollah said several others were wounded in the blast and that it was investigating the nature of the explosion — whether it was the result of an air raid, missile attack or artillery shelling.

It didn’t say when the explosion happened, and Hezbollah’s media office said they also had no information about the timing of the attack. On Tuesday, Hezbollah denied reports that the Israeli Air Force targeted a Hezbollah convoy on the Lebanon-Syria border.

Badreddine was one of four people being tried in absentia for the murder of former Lebanese prime minister Rafik Hariri. The 2005 Valentine’s Day suicide bombing that killed Hariri and 22 others was one of the Middle East’s most dramatic political assassinations. The trial is ongoing near The Hague, Netherlands. A billionaire businessman, Hariri was Lebanon’s most prominent politician after the 15-year civil war ended in 1990.

Hezbollah denies involvement in Hariri’s assassination and says the charges are politically motivated.

One of the group’s most shadowy figures, Badreddine was also known by aliases Elias Saab and Sami Issa. He was only known to the public by a decades-old black-and-white photograph of a smiling young man wearing a suit until Hezbollah released a new image of him in military uniform.

The US Treasury Department imposed sanctions twice on Badreddine for his involvement in the Syrian war, in 2011 and in 2015. According to US officials, Assad and Nassrallah coordinated Hezbollah’s actions in Syria on a weekly basis, with Badreddine present at top Damascus meetings.

Badreddine was also known for his expertise in explosives, apparently developing what would become his trademark explosive technique by adding gas to increase the power of sophisticated explosives.

The Hezbollah commander was suspected of involvement in the 1983 bombings of the US and French embassies in Kuwait that killed five people. He was detained in Kuwait where he was sentenced to death and imprisoned for years until he fled jail in 1990 after Iraq’s Saddam Hussein’s forces invaded Kuwait.

Since Hezbollah was founded in 1982, Israel has killed some of the group’s top leaders. In 1992, Israeli helicopter gunships ambushed the motorcade of Nasrallah’s predecessor, 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.

In December, high profile militant Samir Kantar, who spent 30 years in an Israeli prison, was killed along with eight others in an airstrike on a residential building in Jaramana, a Damascus suburb.

There was no immediate comment from Israel on Badreddine’s death. Former national security adviser Yaakov Amidror said Friday that Badreddine’s death was good news for Israel, whether or not the Jewish state was behind the killing.

Hezbollah has paid a very steep price for its public and bloody foray into Syria’s civil war, where more than 1,000 fighters have been killed. Once lauded in Lebanon and the Arab world as a heroic resistance movement that stood up to Israel, his staunch support for Assad has been criticized at home, even among his Lebanese support base.

The Arab League designated Hezbollah a terrorist organization in March. A month earlier, Saudi Arabia cut $4 billion in aid to Lebanese security forces after Lebanon’s Foreign Minister Gibran Bassil declined to join Arab and Islamic league resolutions critical of Iran and Hezbollah.

The predominantly Sunni Gulf Arab states, led by the kingdom, have taken other punitive measures. They have warned their citizens against traveling to Lebanon as well as cut Lebanese satellite broadcasts, and closed a Saudi-backed broadcaster in Lebanon. The Gulf countries are also expelling Lebanese expatriates they say have ties to Hezbollah.

Hezbollah, which maintains a dominant militia force in Lebanon, has also aligned itself with the Saudi-opposed Shiite Houthi rebels in Yemen’s civil war.

Hezbollah’s statement quoted Badreddine as saying in Syria a few months ago: “I will only return from Syria as a martyr or carrying the banner of victory.”

Copyright 2016 The Associated Press.