Peter Lawrence: 'We just miss you, we just want to know that you're safe'
The disappearance of York University chef Claudia Lawrence is now being treated as suspected murder, North Yorkshire Police said.
However detectives said they had not found any proof that the 35-year-old, who went missing on 18 March, was dead.
Her father Peter Lawrence made a direct appeal to his daughter to contact him five weeks after she disappeared.
His plea came at a news conference held shortly after a £10,000 reward was offered to help find Miss Lawrence.
Crimestoppers said the sum they were offering was "significantly higher" than usual because of public interest in the case.
Please, please listen to these messages and if you're able to, please send just a quick text or make a quick call
Det Supt Ray Galloway, who is leading the investigation, said: "At the moment we have no proof of Claudia's death.
"However five weeks on from her disappearance we also have no proof of her life.
"The investigation is now being formally classified as one of suspected murder."
Officers now want to speak to a man and woman seen on Melrosegate Bridge at 0535 GMT on Thursday, 19 March.
Detectives believe the two people may have useful information.
In his appeal to his daughter, Mr Lawrence said: "It's just so unusual for you not to be in contact that we just don't know what is happening.
"Despite many appeals, nobody's heard from you, and we're obviously extremely worried."
Mr Lawrence said both he and Claudia's mother had left new messages on their daughter's mobile phone and urged her to listen to them.
He said: "Please, please listen to these messages and if you're able to, please send just a quick text or make a quick call.
"You may not be able to do this but we just hope there's that possibility that you can.
"There's absolutely nothing for you to worry about in terms of getting in contact, you're not in trouble with anybody. We just miss you, we just want to know that you're safe."
On Thursday, North Yorkshire Police said they had scoured thousands of rooms at York University and searched hundreds of homes.
Detectives said they had taken 868 reports and statements and handled more than 900 telephone calls.
Concerns have been raised that potential leads in the search were not being followed up quickly enough.
North Yorkshire Police said leads were assessed methodically and carefully.