Syria's interior ministry said on Sunday that 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 the attack on Saturday, which the US Central Command said had been carried out by a Daesh gunman 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 once occupied by Daesh during the height of its terror campaign in Syria.
The incident is the first of its kind reported since longtime Syrian ruler Bashar al Assad was overthrown in December last year, and the country's ties with the United States were rekindled.
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".
US President Donald 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 attack were "doing well".
