JIBLA, Yemen A suspected Muslim extremist, cradling his hidden gun like a baby under his jacket, slipped into a Southern Baptist hospital in Yemen on Monday and opened fire, killing three American missionaries and seriously wounding a fourth, officials said. The suspected attacker, a Yemeni, was arrested, and a Yemeni official said security forces were searching for a cell that may be targeting foreigners and prominent Yemenis that Islamic extremists deem too secular. Americans have been repeatedly warned by the State Department be cautious in Yemen, a country where central government authority is weak in tribal areas, guns are plentiful and Muslim militants have found refuge. Yemen, the ancestral homeland of al-Qaida chief Osama bin Laden, has been a key front in the U.S.-led war on terrorism. The White House condemned the attack in the town of Jibla, but said it was too soon to tell if it was linked to terror groups. U.S. investigators were working closely with the Yemenis to bring to justice all those who are responsible, spokesman Scott McClellan said. The gunman entered the complex of Jibla Baptist Hospital, slipping past a security check where visitors are supposed to leave their weapons by hiding his semiautomatic rifle under his jacket to make it resemble a child, officials and the missionary organization said. Its not uncommon in Yemen for children to be carried under loose fitting clothes. The attacker entered a room where hospital director William E. Koehn was holding a meeting and opened fire, said the Southern Baptist Conventions International Mission Board, based in Richmond, Va. Koehn and two others were killed instantly with shots to the head, Yemeni officials said. The gunman then went to the hospitals pharmacy and shot and wounded the pharmacist, Donald W. Caswell. Koehn, 60, of Arlington, Texas, had planned to retire in October after 28 years of service. The Mission Board identified the other two dead as purchasing agent Kathleen A. Gariety, 53, of Wauwatosa, Wis. and Dr. Martha C. Myers, 57, of Montgomery, Ala. Caswell, 49, of Levelland, Texas, was shot in the abdomen and hospital officials said he was in critical condition. His father, D.C. Caswell, said his son was recovering after surgery. The killings are a crime unacceptable in any religion. This contradicts Islam, said a Jibla woman who gave only her first name, Fatima, and said she used the hospital. They cared for us and looked after us. I cant even count the number of children they treated and saved. After the attack, a Yemeni military jeep with a soldier behind a large machine gun was posted outside the gates of the hospital, a compound of several of one-story, corrugated tin-roof buildings in Jibla, 125 miles south of Sana. Yemeni President Ali Abdullah Saleh sent a message to President Bush, condemning the shootings as criminal and disgraceful and pledging to punish the perpetrators, the official news agency, Saba, reported. Salehs government has cooperated with Washingtons in the war on terrorism. In Sana, the capital, U.S. Ambassador Edmund J. Hull told The Associated Press that U.S. officials did not envision a general evacuation, but we will assist American citizens in Jibla if they wish to leave. About 30,000 U.S. citizens, most of Yemeni origin, live in the country, according to the embassy. The U.S. Embassy in Sana urged Americans to enhance their security. Jack Graham, president of the Southern Baptist Convention, called the three victims martyrs who were killed in the line of duty. Speaking from Plano, Texas, Graham said that aside from providing humanitarian aid, the missionaries were there because theyre Christians and they have no doubt been sharing their faith. Mission Board president Jerry Rankin said his organization would continue to operate in Yemen. He said there had been threats against his groups missionaries though not specifically against the hospital. The threats are taken seriously, he said. It goes with being a Christian missionary now, but also with being an American. It was the second recent attack on American missionaries in the region. On November 21, a gunman shot and killed an American missionary nurse in the Lebanese city of Sidon. Lebanese authorities have yet to determine who was behind that shooting. A Yemeni Interior Ministry official identified the 30-year-old assailant in Mondays attack as Abed Abdul Razak Kamel, the official news agency Saba reported. Officials said they believed he was a Muslim extremist. Kamel said during interrogation that he plotted the shooting in collaboration with Ali al-Jarallah, who was arrested for killing a senior Yemeni leftist politician on Saturday, an official said. Another security official, also speaking on condition of anonymity, said authorities were searching for a five to eight extremists targeting foreigners and Yemeni politicians or other figures, who like the slain leftist, do not adhere to Islamic fundamentalist ideology. The Missionary Board said its 80-bed Jibla hospital treats more than 40,000 patients annually, and its missionaries also teach English and clinical skills at a nearby nursing school. Impoverished, factionalized, predominantly Muslim Yemen has for years been a haven for wanted Muslim extremists. Bin Laden enlisted thousands of Yemenis to fight alongside the mujahedeen of Afghanistan in their U.S.-backed war against an occupation Soviet army in the 1980s. Many have since returned. On Oct. 6, an explosives-laden boat rammed a French oil tanker off Yemens coast, killing a crewmember. U.S. intelligence officials suspect militants linked to al-Qaida in the attack. Two yeas earlier, a suicide bomb boat hit the USS Cole in the southern port of Aden, killing 17 sailors in an attack blamed on al-Qaida. Al-Qaida also is held responsible for the Sept. 11, 2001, attacks in the United States.