Two people are being held over an acid attack on two British teenagers in Zanzibar.
Five people had been arrested, with three being later released, Sky News reports.
The girls have been named locally as Katie Gee and Kirstie Trup. Today images of the burns sustained by one of the girls were released.
The volunteer teachers, both 18, were attacked late at night as they walked through the streets of Stone Town, the historic centre of the capital of the east African island.
They were reportedly targeted by two men on a moped on Wednesday night.
The teenagers are from London and were in the last week of a trip organised by Kent-based i-to-i Travel when men on a moped reportedly threw a corrosive substance at their heads and chests.
A British tourist who gave his name as Sam, tried to help the pair after the attack. He told the BBC: "We heard screaming that sounded like it could be something bad or it could be people messing around. And it sounded like... not panicky screaming, so at first it [made the] hair at the back of your neck stand up and then it was completely fine.
"And then maybe 15 seconds later there was a sort of bigger scream."
The attack took place in Stone Town, an area of the Tanzanian archipelago's capital city, the travel company said.
In a statement, i-to-i Travel said they had been released from a hospital in the Tanzanian capital, Dar es Salaam, where they had been flown for medical treatment after the "acid attack".
"All our efforts remain focused on ensuring they are supported whilst assisting them and their relatives with the arrangements for their return home.
"i-to-i Travel is working with the authorities in their investigations and continue to liaise closely with the British consulate in Tanzania.
"The motive for the incident is as yet not known and we will await the report from the local authorities in Zanzibar before any comment can be made.
"The female clients had been volunteer teachers on Zanzibar and were in the final week of their trip.
"The safety of our customers is of paramount importance to us and our own investigation will be launched as soon as it is possible to do so and more information will be released as it becomes available."
Zanzibar is an archipelago of islands in the Indian Ocean around 22 miles off the coast of mainland Tanzania.
The semi-autonomous region is predominantly Muslim and has been the scene of some religious violence in recent years.
In November last year a cleric was hospitalised in an acid attack and two Christian leaders were killed earlier this year.
i-to-i Travel, which is based in Tunbridge Wells, Kent, runs gap year trips to parts of Africa, Asia, central and South America and Australia, with trips to teach in Zanzibar, based in Stone Town, starting at £669 for two weeks, according to its website.