Two British teens who were victims of a brutal, unexplained acid attack on the East African island of Zanzibar are back home for medical treatment.

Katie Gee and Kirstie Trup, both 18 and from London, arrived on a medivac plane and were taken to the burn unit of Chelsea and Westminster Hospital by ambulance Friday, Sky News reported.

The teens were splashed with acid on their faces, chests, back and hands by two men on a motorbike while walking through the historic capital Stone Town Wednesday night.

Zanzibar police officials told Sky that five people were questioned in the attack. Three suspects were released but two have been detained for further questioning.

Sky News Foreign Affairs correspondent Lisa Holland report there were suggestions that one of the people held was a local shopkeeper who had been in an altercation with one of the girls a few days before the attack.

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    The motive for the assault remains unclear. Other eyewitnesses say the acid was not intended for the girls, but rather someone else walking on the same street.

    The teens were enjoying their last week on Zanzibar after volunteering as teachers at an orphanage.  Both were planning to start college in the fall.

    Gee’s mother, Nicky Gee, told Sky her daughter had suffered burns to her “whole face and body” in the attack, which she described as “vicious.”

    Strup also reportedly suffered horrific burns and was said to be “inconsolable.”

    The volunteer program the girls were on, i-to-i Travel, issued a statement saying it was working with local authorities and welcomed the investigation into the incident.

    "We wish the girls a fast and full recovery and our thoughts are with them and their families at this difficult time," the statement said.

    Zanzibar is an archipelago of islands in the Indian Ocean about 22 miles off mainland Tanzania.The semi-autonomous region of Tanzania is predominantly Muslim and has seen some religious violence in recent years.

    The program said it had advised all members still in Zanzibar "to remain vigilant" and anyone due to fly there this weekend had been given the option to volunteer at a different project, but all had chosen to travel to Tanzania.

    Earlier this year two Christian leaders were killed and in November a cleric was treated at a local hospital following an acid attack. This is the first such assault on foreigners in the popular tourist destination.

    Zanzibar police continue to investigate and have offered a reward of more than $6,000 for any information leading to an arrest.

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