Two British women who had acid thrown over them in Zanzibar arrived back in London yesterday and police on the semi-autonomous Tanzanian islands said they were holding two people for further interrogation.

Police in Zanzibar, a coral-fringed Indian Ocean archipelago, have offered a 10 million-shilling (€4,620) reward for information leading to the arrest of more suspects.

The motive for the attack is still unknown.

Two men on a motorbike threw acid on the British volunteer teachers Katie Gee and Kirstie Trup, both 18, on Wednesday night as they walked to a restaurant in Stone Town, a World Heritage site popular with foreign tourists.

There has been no claim of responsibility for the attack and Zanzibar police said there was no evidence to suggest the involvement of Uamsho, a local Islamist group political analysts say is taking an increasingly hardline religious leaning.

“We have so far questioned five people over the acid attack. Three have been released and two are still being detained for further questioning,” Zanzibar Urban West regional police commander Mkadam Khamis Mkadam told Reuters.

“We believe they were at the area when the attack occurred.”

The two women were flown into a military air base in west London yesterday, Sky News television showed.

In a sign of the potential risks to Tanzania’s image abroad, Tanzanian President Jakaya Kikwete visited the pair in hospital and promised to hunt down the suspects.

Tourism is the second-biggest foreign-exchange earner in Tanzania after mining and any lasting damage to the country’s reputation abroad could hurt industry revenues.

Police Commander Mkadam said the attack could have been the work of common criminals.

Acid attacks are becoming increasingly common in Tanzania, police said.