The shooter who ambushed U.S. and Syrian troops, killing two American soldiers and one civilian who was serving as an interpreter, is believed to have been an Islamic State infiltrator working as part of a local security force, multiple sources told CBS News.

The shooting happened during a mission by U.S. and Syrian forces in a historic central town near Palmyra on Saturday. President Trump and U.S. military officials attributed the attack to ISIS. Two members of Syria's security force were wounded.

Sources told CBS News that the local security force was in the Badia, which falls under the control of the Syrian government.

U.S. Central Command said in a statement on Saturday that the "gunman was engaged and killed." Mr. Trump on Saturday vowed that the U.S. would retaliate.

"This was an ISIS attack against the U.S., and Syria, in a very dangerous part of Syria, that is not fully controlled by them," Mr. Trump wrote in a post on Truth Social. "The President of Syria, Ahmed al-Sharaa, is extremely angry and disturbed by this attack. There will be very serious retaliation."

On Sunday, Mr. Trump said, "We had three great patriots terminated by bad people, and not the Syrian government." Mr. Trump also vowed, "There will be a lot of damage done to the people that did it."

Secretary of State Marco Rubio spoke on the phone with Syrian Foreign Minister Asaad al-Shaibani on Sunday, the State Department said. According to a statement, al-Shaibani "offered condolences and reiterated the commitment of the Syrian government to degrade and destroy the shared threat of ISIS."

No group has claimed responsibility for the attack, and the identity of the gunman has not been released.

Syria's interior ministry spokesman Noureddine al-Baba told state television on Sunday that Syrian authorities "had decided to fire him" from the security forces before the attack for holding "extremist Islamist ideas" and had planned to do so on Sunday. A Syrian security official told Agence France-Presse on Sunday that "11 members of the general security forces were arrested and brought in for questioning after the attack."

Director of the National Counterterrorism Center Joe Kent called the shooting "an insider terrorist attack."

Saturday's attack was the first with fatalities since the fall of Syrian President Bashar Assad a year ago.

The U.S. has hundreds of troops deployed in eastern Syria as part of a coalition fighting the Islamic State group and to hinder Iranian influence. Currently, about 900 troops are stationed there. They are mainly posted in the Kurdish-controlled northeast and at the al-Tanf base in the southeastern desert near the borders with Iraq and Jordan.

Palmyra, home to UNESCO-listed ancient ruins, was controlled by ISIS at the height of its territorial expansion in Syria.

The U.S. had no diplomatic relations with Syria under Assad, but ties have warmed since the fall of the five-decade Assad family rule. The interim president, Ahmad al-Sharaa, made a historic visit to Washington last month, where he held talks with Mr. Trump.

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