British detectives have been allowed to join the hunt for two men who threw battery acid on teenagers Kirstie Trup and Katie Gee in Zanzibar last August.
The acid was thrown on the British girls, both 18, from a motorbike on the island.
Kirstie's father Marc, 51, told the Daily Mirror: "This is a big step. A letter of assistance is coming to the UK which will allow the British entry into the investigation and move the thing forward."
The BBC reported in September that 15 people were arrested in connection with the attack and Zanzibari officials offered a reward but no-one has been charged since.
Police in Zanzibar were also criticised for failing to interview witnesses.
Shortly after the attack, Simai Mohammed Said, of the Zanzibar Association of Tourism Investors, told the Daily Telegraph: "The police need to interview everyone, all the girls' friends here, any potential witness, it's a large number of people they should be talking to.
"As the days go by, the culprits have more and more time to run, and it's more likely that tourists who helped the girls when they were injured and could give witness testimony will leave the island."
In June 2013, a woman died after a month of enduring horrific injuries following an acid attack at a Mumbai train station in India.
Preethi Rathi, 24, was attacked as she alighted from the New Delhi-Mumbai Garib Rath Express in Mumbai with her father.