More than twice as many U.S. service members may have been injured in recent attacks in Iraq and Syria than the Pentagon previously disclosed, U.S. defense officials said on Monday. At least 45 Americans are reporting minor injuries or potential traumatic brain injuries.

The military is monitoring approximately two dozen service members who may have suffered traumatic brain injuries (TBI) during the attacks, which are believed to have been carried out by groups with links to Iran, according to U.S. defense officials.

The Defense Department has previously said 21 service members received minor injuries in attacks on U.S. forces at al-Tanf in southern Syria and on al-Asad air base in western Iraq late last month. The officials attributed the increase to more reports of possible traumatic brain injuries from those attacks. The number of possible TBI cases is likely to change over the coming weeks and months as troops who report mild symptoms initially could be cleared or as more individuals with symptoms could come forward.

Three members of Congress who are all military veterans sent a letter Monday asking Defense Secretary Lloyd Austin how the Pentagon is protecting service members stationed at forward operating bases from traumatic brain injury. 

“The Department must proactively work to reduce the risk to service members both to protect our men and women in uniform and to preserve the capability and readiness of forward operating bases,” Reps. Ruben Gallego, D-Ariz.; Morgan Luttrell, R-Texas; and Bill Johnson, R-Ohio, wrote. 

They also asked for more information about how the military screens for traumatic brain injury after incidents to identify and treat it soon after the injuries occur.

Since Oct. 17, there have been at least 38 separate attacks on bases housing U.S. troops in Syria and Iraq, mainly by one-way drones, mortars or rockets. The two attacks that caused the injuries were both on Oct. 18. 

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One of the attacks, on al-Tanf in southern Syria, included two drones. One of them was shot down, but the other struck the base, injuring U.S. troops. The other attack, on al-Asad air base in Iraq, also involved two drones. Both were shot down, but one broke apart over the base, with the debris destroying a hangar and injuring the troops inside. 

A U.S. contractor died that day when he suffered a heart attack sheltering in place at al-Asad, the Pentagon said. The injuries so far include at least 32 people at al-Tanf, 13 at al-Asad and one in Erbil in northern Iraq, according to Pentagon press secretary Brig. Gen. Patrick Ryder.

A separate attack on Oct. 25 had the potential to be catastrophic for the U.S. military, two U.S. defense officials said. An explosives-filled drone hit the roof of a building housing U.S. troops on a base in Erbil. The drone did not explode. If it had, dozens of troops inside could have been injured or killed, the officials said.

The Wall Street Journal first reported that the drone landed on the barracks.

Last month, the U.S. military conducted strikes on two targets in eastern Syria to retaliate for the recent spate of attacks, hitting an ammunition storage facility and a weapons storage facility that the U.S. said had direct ties to Iran’s Islamic Revolutionary Guard Corps and its proxy groups. 

The U.S. blames the recent attacks on Iranian-backed militia groups. While they have not said Iran is directing the attacks, U.S. officials say Iran is responsible for funding, arming, equipping and training the groups.

“What we want is for Iran to take very specific action to direct its militias and proxies to stand down,” a senior U.S. defense official said after the strikes last month.