Seven paramedics from the White Helmets rescue organisation in Syria have been shot dead during a raid by unknown gunmen in the province of Idlib.
The massacre has sparked a manhunt and speculation as to who carried out the killings at the volunteer group's base in the rebel-controlled village of Sarmin.
The victims' bodies were left sprawled across the floor of their operations centre after being shot through the head at close range.
They were buried by their colleagues later that day.
“The heart is saddened, there are tears in our eyes for your departure,” said the White Helmets' founder, Raed al-Saleh.
“May God strengthen us and make us patient for facing this tragedy.”
The White Helmets, also known as Syria Civil Defence, search for survivors in the rubble of bombed buildings.
They gained international attention following an Oscar-winning Netflix documentary about their work.
Several of their volunteers have been killed by airstrikes during the Syrian civil war, but activists say they have never been the subject of execution-style killings before.
One of the victims was Mohammed Abu Kifah, who was filmed weeping after he pulled a baby girl from a pile of rubble in a viral video last year, according to the Telegraph.
Two vans, several motorcycles and walkie talkies were stolen during the attack, but the motive remains unclear.
Most of the Idlib province in northwest Syria is controlled by the Islamist rebel group, Hayat Tahrir al-Sham (HTS), which has links to al-Qaeda.
However activists don't believe they were behind the murders because they have never targeted the White Helmets before.
Some suspect it was a criminal gang who wanted to steal vehicles and equipment.
Others believe it was agents of the Assad regime, which has accused the White Helmets of working with terrorists.
The UK-based Syrian Observatory for Human Rights said the killings were discovered when volunteers arrived to start a shift and found the bodies of their colleagues.
"It is most likely a crime. It might also be an attack aimed to harm the image of the Nusra Front and to show that Idlib is not safe," said Rami Abdurrahman, who heads the Observatory.
An opposition activist based in Idlib told the Associated Press that the attackers used pistols equipped with silencers, and said that people living nearby did not notice anything unusual.
The activist, who asked that his name not be used for fear of reprisals, said Isis sleeper cells had been discovered in Sarmin.
He added that he thought members of Isis carried out the attack to show that Idlib is not safe.
The activist said the HTS-linked Judicial Committee was investigating the case.
Sarmin used to be a stronghold of the Jund al-Aqsa extremist group that clashed with al-Qaeda last year before many of its members went to areas controlled by Isis, whom they are fighting now.
Elsewhere in Syria, a suicide attacker blew himself inside a training camp for the Army of Islam rebel group in the southern town of Naseeb, near the border with Jordan, killing more than 20 fighters.
The Observatory said that the blast killed 23 people and wounded 20, some of whom were in critical condition.
Ahmad al-Masalmeh, an opposition activist based in the southern province of Daraa, said about 80 Army of Islam members were having dinner inside a tent when the suicide attacker walked in and blew himself up. He put the death toll at 30.
No one claimed responsibility for the deadly bombing but Isis has previously claimed such attacks.
Additional reporting by Associated Press.
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