(CNN) The Marine killed in an ISIS rocket attack in Iraq on Saturday was at the first American firebase that had only become operational a few days earlier CNN has learned.

The existence of the firebase had not been made public. The Pentagon had planned to acknowledge the firebase this week, a defense official tells CNN.

The rugged location for now has a "couple of hundred" Marines living in tents near Makhmour in northern Iraq. It's assumed ISIS observed the Marines moving into the area and saw them firing practice rounds with their howitzers, the official said.

ISIS on Saturday fired two rockets from about 15 km away. One fell inside the base killing one Marine and wounding several others. The Marines returned fire with their artillery.

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