(CNN) The four Americans killed in a suicide blast Wednesday in northern Syria possessed unique expertise for conducting highly classified intelligence-gathering missions in combat zones, according to several defense officials who have spoken to CNN.

The US military has not yet publicly acknowledged why the team had traveled to a busy commercial area in the city of Manbij, which also housed a restaurant that many US government personnel frequented in the past.

In fact, within hours of the attack, the first statement from the US military said the team had been "conducting a routine patrol." But that was followed with another statement that four Americans had been killed "conducting a local engagement," and three additional US troops had been injured.

Several officials told CNN the Pentagon is still assembling details of the mission, and what exactly took the team to that location at that time. But if the military's broad description of the event as a "local engagement" is accurate, it indicates the team was potentially meeting with a sensitive source to gather intelligence or may have even been using electronic equipment to conduct eavesdropping and electronic intercept missions.

As with all attacks, the military is investigating what happened, and how it happened, officials say. Regardless of the exact mission and the reason the group had left the security of the base, there would have been detailed planning and an assessment of the risk at the time they planned to travel.

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