The United States has vowed to hunt down "any and all" of those responsible for gunning down three American Christian missionaries in Yemen.
Yemeni police arrested a 30-year-old man. They said he was an Islamist militant and had told them he attacked the Baptist mission hospital in southern Yemen yesterday to get closer to God.
Witnesses said Abed Abdel Razzak Kamel entered a provincial clinic at Jibla posing as a patient. He is accused of shooting dead a doctor and two of her colleagues. An American pharmacist was wounded.
US officials said it was too early to jump to conclusions about whether the man acted alone or was linked with groups such as al-Qaeda, whose leader, Osama bin Laden, has his family origins in Yemen.
"We strongly condemn and deplore the murder of three American citizens who were providing humanitarian assistance to the Yemeni people," said White House spokesman Mr Scott McClellan.
He declined to label the murders a terrorist attack but said Washington was working closely with Yemeni officials on the case.
"Our intention is to bring to justice any and all people who were responsible for these murders."
The Southern Baptist International Mission Board named the dead as physician Ms Martha Myers (57) of Montgomery, Alabama, hospital administrator Mr William Koehn (60) of Arlington, Texas, and Ms Kathleen Gariety (53) of Wauwatosa, Wisconsin, a purchasing agent.
Wounded pharmacist Mr Donald Caswell (49) of Levelland, Texas, was recovering after an operation to remove two bullets.
The gunman smuggled a rifle into the hospital concealed in his clothing and cradling it like a baby, president Mr Jerry Rankin said at Mission Board's headquarters in Richmond, Virginia.
Mr Rankin said the humanitarian work would go on and that the 35-year-old hospital in Ebb province, 170 km (105 miles) south of the capital Sanaa, would probably stay open.