Police searching for missing Megan Roberts have said the most probable scenario is that she "entered the river" while "heavily affected by alcohol".
Superintendent Phil Cain described the last known movements of the York St John University student almost a week after the 20-year-old went missing in the city.
Mr Cain said: "The strongest and most probable line of inquiry being pursued by police is that Megan, affected by alcohol, has entered the river."
Speaking close to the Lendal Bridge over the River Ouse in York, Mr Cain stressed that there was "no evidence to indicate foul play".
He also said there was nothing to link Miss Roberts' disappearance to the case of Claudia Lawrence - the university chef who went missing in York in 2009 and who police believe has been murdered.
"On balance, this presents as a group of young friends on a night out that is likely to have ended in tragedy," the officer said.
Miss Roberts, who is from Wetherby, West Yorkshire, and lived in the Clifton area of York, was last seen in the early hours of Thursday following a night out.
But it was only when family and friends realised earlier this week that she was missing that the alarm was raised.
North Yorkshire Police have studied CCTV footage and interviewed Miss Roberts' friends in an effort to trace her last movements.
A police diver has been scouring the river bank close to the Lendal Bridge, near where Miss Roberts was last captured on CCTV.
Mr Cain said: "All the group, including Megan, were heavily affected by alcohol."
He explained how the group members were spread out and some ran down a street called Tanner's Moat, past The Maltings pub, where Miss Roberts - the last in the group - was spotted bumping into a row of cycle racks.
"CCTV evidence does not show Megan running back up the same street with the others to rejoin the main group," he said.
Mr Cain said the group of friends has been extensively spoken to and officers are confident that none of them is directly responsible for Miss Roberts's disappearance.
"The possibility still exists that Megan is elsewhere safe and well but, realistically, given the passage of time, such a possibility is increasingly remote," Mr Cain said.