The U.S. military is now treating 64 troops for traumatic brain injury symptoms following Iran's ballistic missile attack on Al Asad Air Base in Iraq earlier this month, defense officials said Thursday.

"The Department of Defense remains committed to providing the American people timely and accurate information about the care and treatment of our service members," said Army Lt. Col. Thomas Campbell, a Pentagon spokesperson.

CNN was first to report the increase. The latest news marks the fourth time the Defense Department has amended its statement regarding the injuries. Of the 64, 39 service members have returned to duty, officials said.

Earlier Thursday, top Pentagon officials said the number of troops with mild TBI or concussion-like symptoms was expected to rise.

"The number is growing," Chairman of the Joint Chiefs Gen. Mark Milley told reporters during a briefing. Medical professionals continue to screen troops involved in the attack for TBI, which presents concussion-like symptoms and often manifest days or weeks later.

"It's not instantaneous," Milley added. "For all the folks that have been diagnosed to date, it's mild traumatic brain injury."

The Defense Department's reporting of the incident has come under scrutiny as officials initially said there were no casualties -- a term used to describe those killed, wounded or incapacitated in some way.

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Despite early accounts that no Americans had been harmed, later media reports said that 11 troops had been hurt in the blast, requiring medical evacuation and treatment at facilities in Germany and Kuwait.

Last week, defense officials confirmed that 34 service members had been diagnosed with concussion-like or TBI symptoms since the Jan. 8 Iranian ballistic missile strike. Following treatment, 17 of the 34 returned to duty, Pentagon chief spokesman Jonathan Hoffman said Jan. 24.

The number then grew to 50, DoD officials said this week.

"Over time, you learn things and you get reports … and as we said, there would likely be more and more," Defense Secretary Mark Esper said Thursday. "I think we did our best to report no casualties."

Iran's attack on Al Asad, as well as another on a base in Erbil the same day, came in retaliation for a U.S. drone strike days prior that killed Iranian Quds Force commander Qasem Soleimani.

Esper said that a U.S. commander in Iraq has requested a Patriot missile battery for reinforcement, "and in this case we support the commander."

The U.S. does not currently have Patriot defenses at either Al Asad or Erbil, where American troops are located. In coordination with other Gulf partner nations, the Pentagon has stationed the missile battery at other locations, such as bases in Saudi Arabia, which officials judged were more likely to be in Iran's crosshairs, The Associated Press reported Wednesday.

"[But] we need the permission of the Iraqi government; that's one issue," Esper said.

"There's mechanical pieces -- the 'science of war' so to speak -- of actually moving and bringing in Patriot. So the mechanics need to be worked out [to bring in an Army battalion]," Milley said.

He could not say for certain whether a Patriot battery could have prevented or intercepted the dozen or so ballistic missiles that hit Al Asad. "But it is what they're designed to do," Milley said.

-- Oriana Pawlyk can be reached at oriana.pawlyk@military.com. Follow her on Twitter at @Oriana0214.

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