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.
Today a police diver was scouring the river bank close to the Lendal Bridge, near where Miss Roberts was last captured on CCTV.
Mr Cain said: "Understandably there has been growing and grave concerns for her safety which has triggered a great deal of police activity and interest from the media and the public.
"Some of that activity has played out on social media and has led to a number of theories about her fate, including potential links to the Claudia Lawrence case.
"It is very important, not least for Megan's family and friends, to deal with the facts as they currently present.
"Those facts are based on evidence resulting from a variety of police inquiries to date."
The officers said Miss Roberts was with a group of friends who had been drinking "for several hours" before leaving the Popworld nightclub, in York city centre, at around 2.10am on Thursday morning.
He said: "All the group, including Megan, were heavily affected by alcohol."
Mr Cain explained how the group were spread out and some of them 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.
Yesterday, police played down Twitter messages exchanged between Miss Roberts and friends a few days before she disappeared which appeared to indicate a possible stalker.
Officers said "f riends are convinced there was nothing to suggest Megan was being stalked at that stage" and "we do not believe that is connected to Megan's disappearance".
Miss Roberts is described as around 5ft 6in and of medium build with long dark brown hair. When last seen she was wearing a cream knee-length dress which had large brown and orange flowers on it and black plimsolls-type shoes.
Police said that specially trained officers are supporting the missing student's family.
Volunteers from the charity Missing People were out in force on Lendal Bridge and in the main shopping areas of York today, asking the public to keep vigilant and distributing Miss Roberts' picture.
Martin Crosby, from the charity, said: "When somebody goes missing, that affects the town or city they were from as well as people directly affected. People in York have been very, very generous and very supportive. They are looking to work out how they can help, really."