to focus on military-specific activities; providing supplemental expertise in specialized fields, such as linguistics or weapon systems maintenance; and providing a surge capability to quickly deliver critical support functions tailored to specific military needs. Some have accordingly argued that meeting immediate personnel needs through surges in contractors used by the federal government is more cost-effective on a long-term basis and just as the effective use of contractors can augment military capabilities.

"The timing of the murder of an American citizen in Baghdad puts question marks," al-Sudani said on Monday, adding: "Security is a red line."

Two armed people attacked a vehicle Troell was driving in downtown Baghdad, security sources told CNN. Troell sustained severe injuries in the attack and was transferred to a nearby hospital to receive medical care, but later succumbed to his injuries.

There was no immediate claim of responsibility for the killing.

The Iraqi Foreign Ministry said that investigations into the attack are ongoing by security authorities in Baghdad.

Troell had been living in Baghdad for two years and had worked for a civil society organization that taught English to Iraqis.

"With great sadness and sorrow, we bid farewell to our dear, Stephen Troll, who has always loved Iraq and its people and sought to serve them," Global English Institute Baghdad, where Troell worked, said in a statement on Tuesday.

US Ambassador to Iraq Alina Romanowski expressed her thanks on Twitter Tuesday to "the Iraqi people for their supportive messages following the brutal murder of Steven Troell last night in Baghdad."

"He was here in a private capacity doing what he loved -- working (with) the Iraqi people. My deepest condolences to his wife & young children," Romanowski wrote.

The State Department spokesperson said US officials "stand ready to provide all appropriate consular assistance" following the incident.