Two men received serious facial injuries after being stabbed in Silsden town centre early on New Year's Day.
The men -- both local and in their early 20s -- were taken to Airedale Hospital following the incident, which happened in Kirkgate at about 3am.
One required 27 stitches in a facial wound.
Police are appealing for witnesses to the attack, or anyone who has information, to come forward.
They are also keen to hear from anybody who may have found the weapon, believed to be a long-bladed knife.
The incident, thought to involve four people, occurred in the area close to Silsden Post Office and the NatWest Bank.
Det Chief Insp Roger Gasson (pictured), head of Keighley CID, said: "This was an extremely serious incident, which resulted in nasty injuries. We understand that one of the injured men was attacked as he went to the aid of his colleague.
"This all happened by a main road at a time when I am sure there would be people around, or motorists passing.
"We would like to hear from anyone who saw a fracas involving four people, or has information about the incident.
"Also, the weapon used is still missing. If anybody has found a long-bladed knife, please get in contact with us."
Both the injured men were discharged from hospital following treatment.
A 25-year-old man has been arrested in connection with the incident and released on police bail pending further inquiries.
Anyone with information should contact Keighley CID on 01535 617083.
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