Seven member of the Syrian White Helmets rescue group were shot dead by unknown gunmen on Saturday, sparking a manhunt and deep suspicions in the rebel-controlled province of Idlib.
The men were killed in their operations centre in the village of Sarmin at dawn on Saturday. Two of their vehicles as well as several of their distinctive white helmets were stolen, the group said.
Many White Helmet volunteers have been killed in airstrikes during the six year Syrian war but opposition activists said it was the first time that members had been shot dead like this.
Photographs from the scene showed the volunteers' bodies lying across the floor of the centre and blood splashed over the logo of the White Helmets group. All had been shot execution-style at close range.
"The heart is saddened, there are tears in our eyes for you departure," said Raed al-Saleh, the founder of the White Helmets. "May God strengthen us and make us patient for facing this tragedy."
One of the men killed was Mohammed Abu Kifah, who appeared in a widely-shared video last year where he wept with emotion after rescuing a baby girl from the rubble of a bombed-out building.
The White Helmets have been lauded by Western countries for saving civilian lives in opposition areas in Syria and in 2016 they narrowly missed out on winning the Nobel Peace prize.
But the group also operates in areas controlled by jihadists and supporters of the Assad regime regularly accuse them of allying with terrorists.
Idlib province is now mainly controlled by Hayat Tahrir al-Sham (HTS), an Islamist rebel group that is dominated by jihadists linked to al-Qaeda.
Activists in Idlib said they were baffled by the killings but did not think that HTS was responsible. The group has not targeted White Helmets in the past.
One theory was that agents of the Assad regime had killed the men to create suspicion and distrust between rival rebel groups.
HTS recently fought an open battle against Ahrar al-Sham, another prominent rebel group, and there is lingering distrust in Idlib as a result.
"The ones who did this are clever. They want to make conflicts worse between the rebel groups and show that there is no security and no safety in the opposition areas," said Abdulkafe al-Hamdo, an English teacher in Idlib.
Another theory is that the killings were the work of a criminal gang, who wanted to steal equipment from the White Helmets centre. Two vans were taken from the centre as well as several motorcycles and some walkie-talkie equipment.
One of the vans was later found burned out on the edge of a field several miles from the centre.
The White Helmets centre is close to several other houses in the village of Sarmin but neighbours said they had not heard any gunfire during the night. That raises the possibility the White Helmets were killed with silenced weapons.
The Islamic State of Iraq and the Levant (Isil) has released videos in the past showing its fighters using guns equipped with silencers. The Assad regime also has access to silencers through its allies in Russia and Syria.
Mr al-Hamdo said members of the group had last been seen on Whatsapp at around 1am, suggesting they were killed sometime after that.
Gareth Bayley, Britain's special representative for Syria, condemned the killings.
HTS has a network of checkpoints around Idlib province and activists said they hoped that the group's fighters would be able to catch the killers.
Idlib is one of the last major opposition strongholds in Syria and faces intense bombardment by the Assad regime and its Russian allies.
Many of the fighters and civilians the opposition areas of Aleppo fled to Idlib after the city fell in late 2016.