Washington — The U.S. is sending additional air defenses to the base in northeast Jordan where three American soldiers were killed in a drone attack on Sunday, according to a U.S. official. 

The outpost that was hit had not been the target of previous attacks and thus its air defenses were not as strong as U.S. bases in Iraq and Syria that have been under constant threat of attack since October, the official said. The air defense system heading to the outpost is designed to intercept drones. 

About 350 U.S. Army and Air Force personnel are stationed at the outpost, known as Tower 22, according to the Defense Department. 

The Pentagon said Monday that the number of wounded had risen to more than 40 after a drone strike hit their sleeping quarters in the pre-dawn hours on Sunday. 

"People were actually in their beds when the drone impacted," Pentagon spokesperson Sabrina Singh told reporters Monday. 

Singh said the attack was inconsistent with prior strikes because it hit living quarters and was early in the morning. The Defense Department is investigating how the drone evaded air defenses, she said. 

"We are trying to figure out how a one-way attack drone was able to evade air defenses and was able to kill three of our service members and injure dozens more," she said. 

Tower 22 did not have the same air defenses as the special forces base located about 15 miles north in al-Tanf, Syria, which has been under frequent attack. The deadly attack on Tower 22 was the first time a drone had landed on the Jordanian side of the border. 

Since Oct. 17, there have been at least 165 attacks on U.S. forces in Iraq, Syria and now Jordan by Iranian-backed groups. 

Eleanor Watson contributed reporting. 

In:
• Jordan
• Syria
• Defense Department

David Martin

David Martin is CBS News' National Security Correspondent.