A man threatened a pensioner with a knife in his own home in Cleckheaton.
The man entered the victim's home in Pyenot Gardens at about 6.15pm on Friday.
He asked the victim, 83, to hand over the keys to a vehicle that was parked on the drive.
When he refused, the man threatened him with a knife and pushed him.
But the victim managed to pull his audible alarm and the suspect made off empty handed, police said.
He was not hurt during the incident.
Constable Tracy Boucher, of Kirklees CID, said; “This has clearly been a frightening and upsetting experience for the victim, who was thankfully uninjured.
“I would appeal to anyone who may have been in the area around Pyenot Gardens during the early part of Friday evening who may have seen anyone matching the suspect’s description or who saw or heard anything suspicious to come forward with any information. Enquiries are continuing.”
The suspect is described as a white man, around 6ft tall in his mid-twenties, and of medium build.
He was wearing a shiny grey wax-type three quarter-length jacket, and dark-coloured jogging bottoms which were possibly tucked in to his socks.
Anyone with any information is asked to contact Kirklees CID via 101, quoting 13170525912.
Information can also be passed to the independent crime fighting charity Crimestoppers on 0800 555 111.