A bus driver may have suffered permanent eye damage after a lit flare was thrown at her.
Avon and Somerset Police said the 53-year-old woman would need ongoing hospital treatment after the incident in Bristol city centre.
A 14-year-old boy has been arrested on suspicion of causing grievous bodily harm with intent.
Police are keen to speak to a woman who was standing at a nearby bus stop at the time.
According to a force statement, the teenager got on to the bus on Union Street on 30 December and lit the flare on the top deck before getting off shortly afterwards, at about 16:40 GMT, and throwing it in the driver's face.
"The victim suffered burns to her eyes and potentially a deterioration in her eyesight which requires ongoing hospital treatment," it said.
"We want to speak to anyone who saw or heard any part of this incident, who haven't yet spoken with the police.
"We especially want to speak to a woman who was stood at the bus stop at the time."
The 14-year-old has since been released under investigation.