Four teenage boys and a 20-year-old man had to be taken to hospital after they were sprayed with ammonia on a street in Cardiff.
Police said officers are continuing to investigate the alleged assault which happened on City Road at around 2:45pm on Sunday.
During the incident, people were seen pouring milk on their faces after being splashed with the substance in broad daylight.
The five were taken to hospital with minor injuries and discharged later the same day.
Police believe those involved are known to each other and are appealing for anyone with mobile phone or dash-cam footage of the incident to come forward.