An elderly man suffered broken ribs after being attacked while walking his dog.
The 74-year-old needed hospital treatment after the incident in Liversedge on January 31.
West Yorkshire Police said he was walking along the Greenway, close to Wormald Street and Melbourne Street, when he was approached by two males on bikes between 4.45pm and 5pm.
They grabbed him from behind, pushed him to the floor and repeatedly kicked him before fleeing the scene on their bikes.
The victim is recovering at home after being discharged from hospital.
Inspector Dorian James said: “This was an incredibly distressing incident for the gentleman involved.
“Since the incident, officers have conducted house-to-house enquiries as well as examining CCTV in and around the area. This is being treated as a serious incident and my colleagues from CID are investigating the matter to identify the perpetrators.
“I would like to encourage members of the public who were in that area on the day of the incident to come forward to the police with any information which may assist with our enquiries.
"I know there will be someone in the local community who has key information that will help us find those responsible.”