Defense Secretary Ash Carter said in a statement that he was “deeply saddened” to learn that an explosion at Bagram Airfield in Afghanistan just after 5:30 a.m. local time resulted in U.S. casualties.

An apparent suicide bomber took the lives of two U.S. service members and two U.S. contractors working on the base, Carter said. The explosion wounded 16 other U.S. service members and one Polish soldier participating in the NATO mission, he added.

“I want to express my sincere condolences to the families of the fallen, and I want to reassure the loved ones of those injured that they are getting the best possible care,”the defense secretary said. “Force protection is always a top priority for us in Afghanistan, and we will investigate this tragedy to determine any steps we can take to improve it.”

“For those who carried out this attack, my message is simple,” Carter said. “We will not be deterred in our mission to protect our homeland and help Afghanistan secure its own future.”

Response teams at Bagram continue to treat the wounded and investigate the incident, Resolute Support mission officials said in a statement.

Statement by Secretary Ash Carter on Bagram Air Base Explosion: I am deeply saddened to learn that an explosion early this morning at Bagram Airfield in Afghanistan has resulted in U.S. casualties. An apparent suicide bomber has taken the lives of two U.S. service members and two U.S. contractors working on the base. The explosion wounded 16 other U.S. service members and one Polish soldier participating in our NATO mission. I want to express my sincere condolences to the families of the fallen, and I want to reassure the loved ones of those injured that they are getting the best possible care. Force protection is always a top priority for us in Afghanistan, and we will investigate this tragedy to determine any steps we can take to improve it. For those who carried out this attack, my message is simple. We will not be deterred in our mission to protect our homeland and help Afghanistan secure its own future.