A suspected corrosive substance was thrown over two boys in a London Tube station, British Transport Police said.
At just after 8pm on Saturday the boys reported having a liquid thrown on them at Elm Park Tube station in Havering, east London.
A search for those responsible is under way and on Tuesday police released CCTV of three people, including a woman, who they want to speak to.
The woman has long brown hair and is seen walking along a Tube platform wearing a white top and carrying a puffer jacket.
Walking beside her is a person wearing a black face covering, baseball cap and a black puffer jacket.
Another CCTV image captures the pair approaching the barriers with a man in a black jacket.
Investigating officer Detective Inspector Marvin Bruno said: “Violence in any form will not be tolerated on the rail network and we would like to reassure the travelling public that our officers are working tirelessly to identify and apprehend those responsible.
“We are really keen to speak to the people in the images, or anyone else who witnessed the incident, as we believe they have information which could help our investigation.
“If you know them or have any information that might help, please get in touch.”
The boys attended hospital and their injuries are not thought to be life-threatening or changing.
Anyone who recognises the three people on CCTV, or who has any information, is urged to contact British Transport Police by texting 61016 or by calling 0800 40 50 40 quoting reference 803 of 24/02/24.