Detectives are making a direct appeal to a couple they believe were in the immediate vicinity of a serious assault in Newmarket Street in Derry on October 22.
Shortly after 2am police received a report of an altercation between a number of men in Newmarket Street.
A man, aged in his 30s, sustained serious injuries to his head after he was assaulted by a man.
He is currently being treated in hospital for his injuries.
Detective Chief Inspector John Caldwell said: “I am appealing directly to a man and a woman who were in the vicinity of the assault to come forward to police as they may be able to assist with the investigation.
"The man was wearing a coat and T-shirt - both dark in colour - and blue jeans while the assault was taking place in Newmarket Street.
"However, directly following the assault, he was seen walking in William Street not wearing his coat.
"At this stage a large tattoo was clearly visible on his left forearm.
“The woman has long, dark coloured hair and was wearing blue jeans, brown shoes and a three-quarter length green coat.
“I would like the couple, or anyone who recognises their descriptions, to contact detectives in Strand Road on 101 quoting reference 209 22/10/2017 as soon as possible. Or, alternatively if someone would prefer to provide information without giving their details they can contact the independent charity Crimestoppers and speak to them anonymously on 0800 555 111.
“Police would like to thank members of the public for their patience and ask them to avoid the area if possible while the investigation into this serious assault continues.”
Earlier SDLP leader Colum Eastwood said he was requesting an urgent meeting with the PSNI after incident. The man injured, who is now in a critical condition in hospital, is a former volunteer of the Foyle Search & Rescue.