The man who attacked Katie Gee and Kirstie Trup on the Indian Ocean island of Zanzibar smiled as he threw acid in their faces.
A friend of the two women who witnessed the attack said the men on the moped had been hanging around the building where Gee and Trup, both 18, had been staying in Stone Town.
Olivia Moore told Channel 4 News: "The two men went past a group of tourists and stopped for the girls. They looked at each other, nodded, and then the man on the back smiled and threw the acid."
Another local, called Abdul, added that the men had made comments to Gee and Trup before the attack and that the women had told them to "go away".
The motive for the attack is still unknown but reports suggest the pair may have been targeted by a radical Islamic group that wants Zanzibar to break away from mainland Tanzania and impose stricter sharia laws.
Police are also investigating if the attack was linked to the previous sexual advances of the men.
Other reports also suggested that the pair had argued with a local shopkeeper in the days before the attack, while Gee was attacked by a Muslim woman two weeks earlier because she was singing during Ramadan.
Both women were flown back to the UK last week and were taken to hospital for treatment for the burns they suffered.
Trup has since been discharged from the Chelsea and Westminster Hospital, but Gee remains hospitalised in a stable condition.
Both are expected to need skin grafts for the burns they suffered to their faces, chests, arms and hands. Gee is believed to have suffered 80% burns to her right arm and 50% to her torso.
Marc Trup told the Daily Telegraph that his daughter managed to limit her injuries by running into the sea, which helped to wash off the acid. He said Gee panicked and ran to a public toilet.
He added that both women were "struggling to come to terms with their burns": "Since their arrival, the enormity of their ordeal is having a devastating effect on them, as is the extent of the injuries."
Her mother, Rochelle, added: "I can confirm that Kirstie has been temporarily discharged until Thursday, when she's back in for a skin graft."
Mohammad Jawad, the doctor who treated acid attack victim Katie Piper, told the MailOnline that Gee and Trup will need psychological support and "good scar management" for their injuries: "The aim is to get the area healed in a normal way and reconstruct what is lost, and of course we are all looking for aesthetically pleasing outcomes," he said.
Radical Muslim cleric Issa Ponda surrendered to police in Tanzania after being shot in the shoulder with a tear-gas canister as he attempted to escape.
It is not believed Sheikh Ponda is directly involved in the attack, but police say he may have inspired the attack through his extremist sermons. He is being treated in hospital and is expected to be arrested for breach of the peace.
Police commander Faustine Shilogile said: "We are no closer to finding who was responsible for this. But this is a serious police investigation, and we are doing everything we can."