BEIRUT -- Hassane Laqees was a major player in the Lebanese Shiite militant group Hezbollah from its inception three decades ago to its current intervention in Syria's civil war. Over the years, he survived several assassination attempts.
But as he parked his car just after midnight Wednesday near an apartment south of Beirut that he sometimes used, he was shot dead at close range. It was a professional-style killing that signaled an escalation in the attacks Hezbollah has faced after plunging into the turmoil in neighboring Syria on the side of President Bashar Assad.
Mr. Laqees's death was a significant loss for Hezbollah, analysts said, and any of the group's primary enemies -- Israel, the Syrian insurgents the group is battling, or their backers, such as Saudi Arabia or Lebanese Sunni militants -- could have had reason to want him dead. Mr. Laqees was variously described as running the group's telecommunications network and working to procure strategic weapons.
Hezbollah is facing "a convergence of hostilities," from Sunni militants, particularly extremists who are increasingly dominant in the Syrian insurgency and consider Shiites apostates, and from its longtime nemesis Israel, said Kamel Wazne, a Lebanese specialist on Hezbollah.
Hezbollah, a close ally of Iran and Syria's government, has sent fighters to aid Mr. Assad's forces in crucial battles, a decision that has enraged Hezbollah's Sunni rivals, who back the Syrian rebels.
The killing fueled fears of an escalation of tit-for-tat violence in Lebanon related to the Syrian conflict, and it happened despite heightened security measures by Hezbollah. The assassination occurred shortly after Hezbollah's leader, Hassan Nasrallah, had accused Saudi Arabia of being behind last month's deadly bombing at the Iranian Embassy in Beirut, another attack on a major ally of the Syrian government.
Hezbollah did not say how Mr. Laqees had been killed but accused "the Israeli enemy" of targeting him and said Israel would have to "bear all the responsibility and ramifications of this vile crime."
Yet at the same time, memorial images circulating on social media framed his death as part of the conflict in Syria, showing him against a backdrop of the Sayida Zeinab shrine near Damascus, Syria, a site revered by Shiites that Hezbollah has helped to defend.
At the funeral in Baalbek, in Bekaa Valley in Lebanon, mourners gathered under umbrellas to watch a procession that placed Mr. Laqees's portrait alongside those of the group's top leaders, a level of ceremony that analysts said spoke to his importance. Mohammed Yazbek, a senior Hezbollah cleric, said at the funeral that whoever was responsible was associated with those responsible for the attack on Iran's embassy.
Two previously unknown groups whose names suggested that they consisted of Sunni militants claimed responsibility for the killing, although it was unclear whether either group existed beyond online statements. One called itself Free Sunnis of Baalbek, a town where Hezbollah support is strong. The other, Ansar al-Sunnah, said in an online statement that Laqees bore direct responsibility for the "massacre" in Qusair, a strategic border town that Hezbollah helped Syrian forces capture.
Israeli officials denied involvement, and Israeli analysts said they believed that Mr. Laqees had been targeted by radical Sunnis in Lebanon as part of sectarian tensions over Syria.
Hezbollah's statement announcing the death, in keeping with the deep secrecy surrounding its military structure and operations, did not spell out what role Mr. Laqees had played in the organization or how senior he was.
But Iran's semiofficial Fars news agency said he had run the telecommunications network, and Haaretz, an Israeli newspaper, said he had played a significant role in obtaining weapons, including long-range rockets that, according to U.S. officials, Israel had bombed several times to prevent them from being delivered from Syria. Hezbollah has blamed Israel for previous attacks rather than point a finger at Sunni militants, in what analysts view as a signal that the group, which is also Lebanon's strongest political movement, does not want to escalate sectarian tensions inside Lebanon.
The Hezbollah statement said that Mr. Laqees had dedicated his life "to the honorable resistance from its first days to his final hours" and noted that he had lost a son in the 2006 war between Hezbollah and Israel.
Hisham Jaber, a retired Lebanese army commander and head of the Middle East Center for Studies and Public Relations, said it was odd that such a senior figure had not had better security precautions and had been staying outside Hezbollah's security zone and traveling without bodyguards. But Mr. Wazne said it was not unusual for senior commanders to keep a low profile.
Al Manar, a Hezbollah television channel, reported from the scene that the authorities were questioning the concierge of the building where Mr. Laqees had occasionally worked and stayed. Footage showed blood on the driver's seat of his car, and the Manar correspondent said footprints suggested that there had been multiple assassins.