Syria's interior ministry said Sunday the gunman who killed three Americans in the central Palmyra region the previous day was a member of the security forces who was to have been fired for extremism.
Two US troops and a civilian interpreter died in what the Syrian government described as a "terrorist attack" on Saturday, while Washington said it had been carried out by an ISIS militant who was then killed.
The Syrian authorities "had decided to fire him" from the security forces before the attack for holding "extremist ideas" and had planned to do so on Sunday, interior ministry spokesman Noureddine al-Baba told state television.
A Syrian security official told AFP on Sunday that "11 members of the general security forces were arrested and brought in for questioning after the attack".
The official who spoke on condition of anonymity said the gunman had belonged to the security forces "for more than 10 months and was posted to several cities before being transferred to Palmyra".
Palmyra, home to UNESCO-listed ancient ruins, was controlled by ISIS at the height of its territorial expansion in Syria.
The incident is the first of its kind reported since the overthrow of longtime Syrian ruler Bashar al-Assad in December last year, and rekindled the country's ties with the United States.
US President Donald Trump vowed "very serious retaliation" following Saturday's attack.
A Syrian defense ministry official told AFP on condition of anonymity that prior to the attack, US forces had "arrived by land from the direction of the Al-Tanf military base" in southeastern Syria, near the border with Jordan.
"The joint Syrian-American delegation first toured the city of Palmyra, then proceeded to the T-4 airbase before returning to a base in Palmyra", the source added.
A Syrian military official who requested anonymity said on Saturday that the shots were fired "during a meeting between Syrian and American officers" at a Syrian base in Palmyra.
However, a Pentagon official speaking on condition of anonymity told AFP that the attack "took place in an area where the Syrian president does not have control."
- Warnings -
Pentagon spokesman Sean Parnell said the soldiers "were conducting a key leader engagement" in support of counter-terrorism operations when the attack occurred, while US envoy to Syria Tom Barrack said the ambush targeted "a joint US-Syrian government patrol".
Trump called the incident "an ISIS attack against the US, and Syria, in a very dangerous part of Syria, that is not fully controlled by them", using another term for the group.
He said the three other US troops injured in the incident were "doing well".
The official SANA news agency said the attack also wounded two members of the Syrian security forces.
Syrian Foreign Minister Asaad al-Shaibani said Damascus "strongly condemns the terrorist attack".
In an interview on state television on Saturday, Syrian Interior Ministry spokesman Anwar al-Baba said there had been "prior warnings from the internal security command to allied forces in the desert region".
"The international coalition forces did not take the Syrian warnings of a possible ISIS infiltration into consideration," he said.
ISIS seized swathes of Syrian and Iraqi territory in 2014 during Syria's civil war, before being territorially defeated in the country five years later.
Its fighters still maintain a presence, however, particularly in Syria's vast desert.
Last month, during Syrian President Ahmed al-Sharaa's historic visit to Washington, Damascus formally joined the US-led global coalition against ISIS.
US forces are deployed in Syria's Kurdish-controlled northeast as well as at Al-Tanf near the border with Jordan.
