The military chief and top commanders of Syrian insurgent group Ahrar al-Sham were killed in an attack on Tuesday in northwestern Syria. The attack, allegedly an explosion, killed Ahrar al-Sham's leader Hassan Aboud.
The Syrian Observatory for Human Rights said that the attack had killed 28 of Ahrar al-Sham's commanders, representing a crippling blow to the rebel group that was once considered one of the strongest in the Syrian conflict.
Ahrar al-Sham has been a key group in the Islamic Front coalition, which it helped create in 2013, uniting various Islamist insurgent groups in the fight against Bashar al-Assad's regime.
With other groups from the Islamic Front, Ahrar al-Sham has been fighting both the Syrian regime and Islamic State militants. It was at one point considered to be one of the strongest insurgent groups in Syria.
Islamic Front issued a statement on the blast, saying that Ahrar al-Sham's leadership had "died through a car bomb while they were meeting in rural Idlib. They died after years of jihad against the Tyrant of the Levant."
However, others have suggested that Tuesday's incident was a gas attack. Abu Baraa, a member of an Islamist group allied to Ahrar al-Sham, told Reuters that a doctor who examined the bodies reported little sign of external injuries. Abu Baraa said the doctor saw bodies frothing at the mouth, with fluid coming from the eyes and noses.
The Islamic State, which was accused of killing another Ahrar al-Sham leader in February, denied any involvement in the recent attack.
Syria analyst Aron Lund, of the Carnegie Endowment for International Peace, who's written extensively on the Syrian conflict, told VICE News that although Ahrar al-Sham has now appointed new leaders, it may now be the case that a lot of its fighters go over to other factions.
"If Ahrar al-Sham now collapse, or are dramatically weaker, or they split, many of their fighters may look for other groups to fight for. Even though they've been fighting the Islamic State for a long time, that does not mean that a lot of the fighters on the ground will obey their leaders command," he said.
"In eastern Syria, which the Islamic State has mostly taken over from other rebel groups along the Euphrates river, we have seen factions — Ahrar al-Sham and even Free Syrian Army groups — threatened with annihilation, and pledge allegiance to the Islamic State and their leaders and fighters, and joining and fighting for the Islamic State," he added.
Lund said the potential demise of Ahrar al-Sham could work strongly to the Islamic State's advantage in Syria.
"Ahrar al-Sham has been one of the Islamic State's most dangerous opponents, partly militarily, because they were and still are a big group with lots of fighters, but also because they were one of the only groups who could challenge them ideologically in kind of a Salafi-Jihaddi radical Islamic camp — they challenged IS for legitimacy in this camp," he said.
But he added Ahrar al-Sham's fighters could drift to other groups. This could be whoever presents the best ideological case, or wherever their friends go, or they might follow other connections — or simply where the funding takes them.
"They need to eat and they need ammunition, and if another group can provide that, you can fight the regime for them," he said.
The killing of Ahrar al-Sham's leaders could also act to the advantage of the Syrian regime, Lund said, as Ahrar al-Sham and its allies within the Islamic Front have been important in at least three fronts against the regime.
"They were leading the attempts to break the soon-to-be siege on rebel held Aleppo by the Syrian army troops," he said. "And now as far as I know, both of the guys who headed that effort are dead. And in Hama, you have the rebel groups advancing into regime-held Christian and Alawite areas. Ahrar al-Sham were really key to that, and their leader there is now dead."
The most interesting and unpredictable, and perhaps important dimension in the long term is what this will do to the dynamics within the insurgency.
"Will this strengthen the non-Islamist, 'moderate' camp, loosely termed the Free Syrian Army, by hurting one of the main Islamist hardline factions?" Lund asked. "Or will this just open up space for the al Qaeda and Islamic State flank of the insurgency to grow?"
Follow Oscar Webb on Twitter: @OWebb