The United States is reportedly investigating the reason why its air defense system was not fully operational during a strike on a US base in Syria.

Iran PressAmerica: US Defense Secretary Lloyd Austin said on Friday that the US military had carried out airstrikes on facilities belonging to groups responsible for the drone attack on a coalition base in northeast Syria, killing one US contractor.

The US military does not know at present why the system did not work as it should and the circumstances are under investigation, the report said, citing two officials.

One of the officials explained that the Avenger missile defense system on the base, named RLZ, may have been experiencing some unexpected maintenance issue, despite the fact that the troops were on high alert given that the groups had conducted 78 attacks in the past two years, US media reported.

Two service members injured during the attack were treated on site, while three others were evacuated to the medical facility in Iraq along with a wounded contractor, a release issued late Thursday by the Pentagon detailed.

US Defense Department did not reveal the identity of the killed person, pending notification of family, one official said, according to the report.

US officials claimed on Friday that Iran-backed militias fired about a dozen of rockets while attacking a second US base in the area, but that strike did not result in casualties.

The US military controls parts of the provinces of Al-Hasakah, Raqqa, Aleppo, and Deir Ez-Zor, where the largest Syrian oil and gas fields are located.

The Syrian government has repeatedly called the US military presence in the provinces an occupation aimed at stealing Syria’s oil.