Washington (CNN) Sixty-four US military personnel have now been diagnosed with mild traumatic brain injury in the wake of the January 8 Iranian missile attack on the al Asad military base in Iraq, according to two US officials with knowledge of the latest information.

That number has increased since earlier this week when the Pentagon reported the 50 diagnosed cases.

More reported cases have emerged in the 22 days since the attack as medical assessments on nearly 250 personnel continue. Of the 64 diagnosed cases thus far, 39 have returned to duty.

Several Pentagon officials have previously told CNN that the number of diagnosed cases is likely to continue to change. Approximately 200 people who were in the blast zone at the time of the attack have been screened for symptoms.

During a news conference at the Pentagon Thursday, Joint Chiefs Chairman Gen. Mark Milley said the increasing number of reported cases stems in part from the fact that the injuries, for the most part, fall into the category of "mild TBI" which takes time for symptoms to manifest.

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