Detectives are re-appealing for information to help trace a missing 26-year-old woman following a possible sighting of her when she disappeared.
Gemma Garn, from St Dials, Cwmbran, Torfaen, was reported missing a month ago after a night out in Pontnewydd.
Her Kia Rio hire car was later found on the Second Severn Crossing.
A commuter said he saw somebody making their way towards the barrier on the bridge on the morning Ms Garn was reported missing.
He was travelling across the bridge and said he saw a lone person at around 0645 GMT on Saturday, 19 December.
Although the witness cannot say whether it was a man or a woman, the person generally fitted Ms Garn's build.
Police are confident that her hire vehicle, a black Kia Rio 5-door saloon, arrived at the location at around 0550 GMT the same morning.
Det Ch Insp Pete Jones said: "Police are appealing for any other early-morning commuters, travelling along the Severn Bridge in the early hours of the morning, to come forward as they may well have seen something significant without realising it.
"Potential witnesses shouldn't feel that it's too late to come forward.
"We and Gemma's family are desperately trying to piece together Gemma's actions that night and any information would be valuable to the investigation."
Ms Garn - who graduated from the University of Wales Institute, Cardiff in 2006 - is white, about 5ft 6in, with short dark hair with blonde highlights, blue/green eyes and a fair complexion.
She was last known to be wearing a black polo neck top underneath a black and white patterned dress, a brown casual jacket, black tights and black high-heeled shoes.
Anyone with information or who saw her vehicle is asked to contact Gwent Police on 101.