A London Underground worker was pushed onto the tracks at one of the network's busiest stations.
British Transport Police released a CCTV image of a man they want to speak to in connection with the incident at Victoria station.
A customer service assistant (CSA) was pushed from a District line platform onto the tracks shortly after 10am on Tuesday.
The victim did not sustain any injuries but was left "understandably shaken", according to BTP.
London Underground managing director Mark Wild said: "This was an appalling and dangerous attack on a member of our staff.
"Everyone has the right to go about their day without fear or intimidation and we do not tolerate any form of physical or verbal assault on our staff or customers.
"We are working closely with British Transport Police to bring the perpetrator to justice."
CSAs work inside Tube stations to help manage the flow of passengers on and off trains, oversee ticket barriers and provide help at ticket machines.
Inspector Paula Jones from BTP said: "This was a very serious incident and we have a number of urgent inquiries under way to find the person responsible.
"I would like to hear from anyone who was at Victoria station at the time of the incident and saw what happened.
"Thankfully, incidents such as this are very rare and I am grateful the victim was not injured. We police a CCTV-rich environment which helps us identify offenders and bring them before the courts."
Anyone with information is asked to contact BTP on 0800 405040 or text 61016 quoting reference 146 of April 10. Alternatively they can contact Crimestoppers anonymously on 0800 555 111.