Three American medical workers were shot and killed, and an American pharmacist was critically wounded, when a gunman opened fire inside a Southern Baptist missionary hospital in southern Yemen Monday.

The gunman cradled his gun like a baby as he slipped into the hospital, officials said.

Yemeni officials arrested a suspected Islamic militant, and security forces were searching for a militant cell that may be targeting foreigners and secular figures in the Arab country.

Americans have been repeatedly warned by the U.S. State Department be cautious in Yemen, where the central government authority is weak in tribal areas and where Muslim militants have found refuge. Yemen, the ancestral homeland of Usama bin Laden, has been a key front in the U.S.-led war on terrorism.

The gunman entered the complex of Jibla Baptist Hospital hiding a semiautomatic rifle under his jacket to make it resemble a child. He walked to a room where hospital director William E. Koehn was holding a meeting and opened fire, said a statement from the Southern Baptist Convention's International Mission Board, based in Richmond, Va.

Three people were killed instantly there with shots to the head, Yemeni officials said. The gunman then headed to the hospital's pharmacy and shot and wounded the pharmacist, Donald W. Caswell.

The Southern Baptist International Mission Board identified the dead as Koehn, 60, of Arlington, Texas, who had planned to retire next year after 28 years of service; purchasing agent Kathleen A. Gariety, 53, of Wauwatosa, Wis.; and Dr. Martha C. Myers, 57, of Montgomery, Ala.

Caswell, 49, of Levelland, Texas, was in critical condition after being shot in the abdomen. His father, 71-year-old D.C. Caswell, said his son was recovering after surgery.

"We just thank the Lord that he is alive," the elder Caswell said from Texas. "He's alert and talking and everything's going to be all right, they're thinking."

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 can't even count the number of children they treated and saved."

Mission Board spokesman Larry Cox said the organization was "devastated by this news" of the attack. "We are moving quickly to minister to family members" in Yemen and the United States, he said.

Board president Jerry Rankin said his organization would continue to operate in Yemen as long as the government allows.

Carrying weapons is common in Yemen, where people often take them openly into offices and public buildings. At the Jibla hospital, 125 miles south of the capital, San'a, guns were supposed to be checked at the door and all those entering are supposed to be searched.

"The man brought in a rifle under his coat as if cradling a baby bringing him into the clinic," Rankin told reporters in Richmond.

He said there had been threats against his group's missionaries, but would not elaborate. "They are taken seriously," he said. "It goes with being a Christian missionary now, but also with being an American that we would be susceptible to threats in many places in the world."

It was the second recent attack on American missionaries in the region. On Nov. 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 Monday's 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 secular personalities in Yemen.

The U.S. Embassy in San'a condemned the attack "on American citizens who have long been providing humanitarian services to Yemeni citizens." It urged the Yemeni government "to bring those responsible to justice."

In a statement, the embassy also asked Americans in Yemen to enhance their security, saying it was requesting additional protection for them and was sending a team to Jibla to help with the investigation.

The Southern Baptist missionary board said its 80-bed Jibla hospital treats more than 40,000 patients annually, providing care free to those cannot afford it. Its missionaries also taught English and clinical skills at a nearby nursing school, according to the board.

Kathleen Gariety had been in Yemen for about 10 years and along with her hospital work had helped educate Yemeni children, said her brother Jerome J. Gariety Jr. of Colgate, Wis.

"She was a wonderful, devoted person," he said. "She loved the children very much."

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 returned when the Soviets withdrew, and they are a powerful political force here.

On Oct. 6, an explosives-laden boat rammed a French oil tanker off the coast of Yemen, killing one member of the tanker's crew, tearing a hole in the vessel and spilling some 90,000 barrels of oil. U.S. intelligence officials suspect militants with links to Al Qaeda in the attack.

In a similar attack in October 2000, a suicide bomb boat hit the USS Cole in the southern port of Aden, killing 17 sailors in an attack blamed on Al Qaeda.

Yemen has signed on as Washington's partner in the war on terrorism launched after the Sept. 11 attacks.

The Associated Press contributed to this report.