LONDON -- The two young volunteers from London had been briefed on how to dress respectfully. They had been warned not to wear anything provocative while teaching on the island of Zanzibar during the month of Ramadan.
So when the volunteers, Kirstie Trup and Katie Gee, both 18, strolled into Stone Town to get dinner last Wednesday, those who know them said they covered up despite the heat. The two men who rode up behind them on a moped gave no reason for targeting them -- the men stopped, smiled and then doused the young women in acid, severely burning their faces, chests and hands, before speeding away.
Ms. Trup has been temporarily discharged from the West London hospital that has been treating the teenagers since their return to Britain on Friday, but she will return for a skin graft on Thursday, her mother, Rochelle, said. Ms. Gee remained in the burn unit of the Chelsea and Westminster Hospital. "Thank you for all your support," she wrote on Twitter, in the first public comment from the women.
No suspects had been arrested by Tuesday afternoon, and the motive for the attack remained a mystery, Musa Ali Musa, Zanzibar's regional police commissioner, said in a telephone interview. But the ordeal raised questions about religious tension that has bubbled up in unexpected pockets of East Africa, and about whether acid attacks -- a particularly nasty type of assault that can leave victims disfigured for life -- may be spreading to places where there is no history of them.
According to one account, a friend said Ms. Trup had been singled out on the island once before. A local woman went up to her angrily and struck her in the face, telling her not to sing in public during the Muslim fast, the friend, Oli Cohen, told The Daily Telegraph.
"They were both extremely shaken up by it," Mr. Cohen was quoted as saying.
The acid attack appears to be the first such assault on a Western tourist or aid worker, said Jaf Shah, the executive director of Acid Survivors Trust International. It was certainly the first such attack on foreigners in Zanzibar, a popular tourist destination about 20 miles off the coast of mainland Tanzania.
With about 200,000 tourists flocking to the island every year, residents are used to seeing Westerners on their beaches. But in remote parts of the island, signs ask visitors to dress modestly.
On the day of the attack, Ms. Trup and Ms. Gee wore long pants and T-shirts, so their shoulders were covered, said Maria Hannis, who spoke to them shortly before they set off to town that day. Ms. Hannis, a team leader at the charity Art in Tanzania, which organized the teenagers' placement as teachers at the St. Monica Catholic nursery school, said they had been "sensible and sensitive."
Ms. Trup's father, Marc, said the two young women had also been careful not to wear anything that might have revealed that they were Jewish.
"We know it's a Muslim country," he told The Times of London. "They were Western girls. Unfortunately, they went out during the month of Ramadan."
Conservative Muslims and Western visitors have long coexisted peacefully on Zanzibar. But in recent months, there have been several violent episodes with religious overtones, mostly targeting Christians. In February, a Roman Catholic priest was shot to death, and a church was burned. Last year, another priest was shot and wounded, and several churches were burned, Mr. Musa said. "It's a very sensitive issue," he said.
In an episode last November, a moderate Muslim cleric was hurt in an acid attack. The cleric, Sheik Fadhil Soraga, has blamed an Islamic group for the attacks on himself and on the two British teenagers. The group, which calls itself Uamsho, Swahili for "Awakening," wants to force strict dress codes on foreign visitors and ban alcohol outside private hotels.
So far, there is no evidence linking Uamsho to the attack on the teenagers.
One friend of the young women in Zanzibar, Olivia Moore, told the broadcaster Channel 4 that the attackers had stopped and smiled at them before throwing the acid.
According to widely reported comments by their families, friends and people on the scene, the teenagers then ran into a cafe on the seafront, screaming for water. Staff members grabbed bottles from a refrigerator and poured water over Ms. Gee, while Ms. Trup ran into the sea, a decision that may have helped to leave her less badly burned, her father said.
With the help of the British consul, the teenagers were eventually taken to a hotel and washed more thoroughly under a shower there. Before returning to London on Friday, they spent a day in a hospital in Tanzania's largest city, Dar es Salaam, where the president of Tanzania, Jakaya Kikwete, visited them, vowing that those responsible would be found.
The minister of tourism, Said Ali Mbarouk, told The Guardian that the episode had "shocked and shamed" his country. Security was increased in tourist areas, and measures were taken to curb the distribution and production of acid. A reward of more than $6,000 has been offered for information that leads to an arrest.
"All our security forces are working very hard in making sure that the culprits are caught," Mr. Mbarouk said.
Acid attacks are not uncommon in some East African countries -- Uganda, for example, reported 29 in 2010 -- but they have been rare in Tanzania, said Mr. Shah of Acid Survivors Trust International.
Historically, such attacks are not specific to Muslim countries, he said. Acid became widely available in the 18th century in Europe and the United States, and more recently, its use in attacks has been most widespread in South Asia. Of the 1,500 attacks recorded by the police, by governments and in the news media every year, two-thirds take place in India, Mr. Shah said. Pakistan accounts for a considerable number, and while no official figures are available for Afghanistan, aid workers and journalists periodically report attacks there.
In some cases, men have been attacked with acid, including the artistic director of the Bolshoi Ballet, who was assaulted in January. But more than three in four acid attacks are against women, Mr. Shah said, and the practice is most widespread in societies where women have limited rights. Common motives include the rejection of marriage proposals or sexual advances.
This article originally appeared in The New York Times.