Police in Zanzibar have arrested 15 people over the acid attacks on two British women and others.
Some have links to al-Qaida and Somali Islamic extremist group al-Shabab, said police commissioner Mussa Ali Mussa.
Earlier police have seized 29 litres of acid from different people, saying they were illegally in possession of it.
Last week a Catholic priest was attacked and badly injured in the fifth acid attack in Zanzibar since November. Last month the two 18-year-old British women doing volunteer work, Kirstie Trup and Katie Gee, were injured when acid was thrown in their faces.
Some questioned the police claime that terror groups may be involved and have a presence in Zanzibar.
Zanzibar is unlikely to have an al-Shabab or al-Qaida presence, said Ahmeid Rajab, the managing director of the Somali satellite television network, Universal TV. "After all, those radical groups never ever use acid to advance their goal."
Police are looking for another excuse to escape blame for failing to arrest real suspects and instead they are going after innocent people - including teachers who possess acid as part of their teaching resources, said Mr Rajab.
The acid attacks are affecting tourism, Zanzibar's main economic activity, tourism. Zanzibar draws visitors from around the world attracted by the archipelago's natural beauty and powdery white sand beaches. Zanzibar is also a melting pot of African, Indian and Arabian cultures and influences.