Police hunting for a missing university student say there is "nothing to suggest" a stalker or any other criminal activity was responsible for her disappearance.
Detectives searching for 20-year-old Megan Roberts are now scouring the river near where the York St John University student was last seen in the early hours of Thursday following a night out with friends.
But it was only when family and friends realised she was missing yesterday that the alarm was raised.
North Yorkshire Police have studied CCTV footage and interviewed her friends in an effort to trace Miss Roberts' last movements.
And they have sifted through messages on social networking sites following a suggestion the student might have been targeted by a stalker.
However, officers have now ruled out any criminal activity, and are continuing to focus on the River Ouse in York.
Superintendent Phil Cain said: "We are exploring with friends who are aware of a Twitter message, which was commenting around a male in a bar where Megan was at the time.
"Friends are convinced there was nothing to suggest Megan was being stalked at that stage. We do not believe that is connected to Megan's disappearance.
"At this moment in time there's no suggestion of any criminality whatsoever and we are treating this as a missing from home."
Specialist teams have been brought in to search the bank of the River Ouse, reviewing CCTV footage which shows the student with a group of friends on one side of the river but absent from the group on the other.
Miss Roberts was reported missing by her mother yesterday morning after a friend raised concerns, North Yorkshire Police said.
A force spokeswoman said she was last seen at around 2am on Thursday in the Popworld bar in George Hudson Street, having gone out with friends on Wednesday evening.
CCTV footage shows Miss Roberts with a group of people on the corner of Rougier Street near Lendal Bridge at around 2.20am. The group were seen a short time later at the other side of the bridge, on Museum Street, but the student was not with them, police said.
Officers are attempting to trace her movements after she was last seen on CCTV.
Police are searching the riverbank in the city centre and and an underwater search unit has been deployed, a force spokeswoman added.
Mr Cain said: "We are very concerned for Megan's welfare and are urging the public to help us establish her movements in the early hours of Thursday 23 January 2014.
"As with all missing persons inquiries we are doing everything we can to locate Megan and make sure she is safe and well.
"Her disappearance and the fact that she has not had any contact with her family and friends for almost five days now is extremely unlike her and adds to our concern.
"Megan's family are understandably very distressed by her disappearance and I would appeal for anyone who has any information at all about where she might be to get in touch with the police as soon as possible."
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.
Among those who have expressed their concern for the Miss Roberts is Peter Lawrence - the father of Claudia Lawrence, the university chef who went missing in York in 2009.
Mr Lawrence's spokesman, Martin Dales, said: "I know Peter's thoughts and prayers are very much with the family and friends of Megan Roberts.
"The 100-plus hours between when she was last seen early last Thursday morning and the reporting today makes it very difficult for those looking for her, whether police, family or friends as the first few hours of being missing are critical.
"We hope that she, and any member of the public with information, will make contact if she can - 116 000 is the freephone number of Missing People who will confidentially pass on messages - and that she will return safely to her family and friends as soon as she can."
No trace of Miss Lawrence has ever been found and detectives believe she has been murdered.