A White House official condemned an attack at an American-run Christian hospital in Kabul on Thursday that killed three American medical staff members and wounded several others.

“The United States condemns the attack today in Kabul that killed three Americans working to provide health care to Afghans,” White House National Security Council spokeswoman Caitlin Hayden said in a statement. “Any such attack on civilians at a hospital is despicable and cowardly. We send our deepest condolences to the families of all those killed and injured.”

The attack took place at 9 a.m. when doctors and nurses were just arriving for work at Cure Hospital, one of several medical facilities operated by a Pennsylvania-based charity. This particular hospital reportedly provides medical care to poor Afghan children.

U.S. authorities are still trying to figure out who is responsible for the attack. Early reports about the incident identified the gunman as an Afghan police officer but other reports described the person as a hospital guard.

“The United States continues to strongly support those in Afghanistan who abhor this violence and are working to build a peaceful, prosperous future for themselves,” Hayden said. “We remain proud of all Americans serving in Afghanistan, working shoulder-to-shoulder with Afghan partners toward our shared goal of a sovereign, stable, secure, and democratic Afghanistan.”