A man suffered an alleged assault by an attacker in possession of a knife in a Hemel Hempstead park.
Officers are appealing to trace a woman who may have witnessed the incident.
It is reported to have happened in the park to the rear of Woodhall Farm Community Centre, off Datchet Close, at 8.30pm on Thursday, September 20.
The victim, a man in his 30s, suffered a cut to his lip and was also punched.
Detective Constable Paula Mowbray, from the Dacorum Local Crime Unit, said: “We are continuing our enquiries into this incident but are particularly keen to trace an elderly woman, who was reportedly in the area at the time and stepped in to help.
“If this was you, we would really like to speak to you as you may have information that is vital to our case.
“If friends or family have spoken to you about an incident like this or similar, we would also like to hear from you.
“We would like to take this opportunity to reassure the community that we believe those involved are known to one another, and that we are not treating this as a random attack.”
Anyone with information should contact DC Mowbray at email@example.com or the non-emergency number 101, quoting reference 41/38783/18. You can also report information online
> A 37-year-old man from Hemel Hempstead was arrested on suspicion of wounding with intent to do grievous bodily harm in connection with this incident. He has been released on bail until Wednesday, October 17.