New leads are being pursued in the search for missing chef Claudia Lawrence as the sixth anniversary of her disappearance approaches.
Detective Superintendent Dai Malyn, of North Yorkshire Police, said his team had made "significant progress" since the force began reviewing the case in 2013 and may make more arrests in the coming weeks and months.
The detective warned anyone who had not yet revealed information about their relationship with Miss Lawrence or the events in the days leading up to her disappearance to come forward or risk being arrested.
Claudia Lawrence has not been seen since 2009
Miss Lawrence was last seen on Wednesday March 18 2009 and was reported missing by her father on Friday March 20 after concerns were raised when she failed to turn up for her 6am shift at York University.
Police believe she has been murdered.
A number of new searches have been carried out since 2013, including a detailed re-examination of Miss Lawrence's house in the Heworth area of York and a fingertip search of the alleyway the leads to the back of the chef's house.
A 60-year-old man was arrested last year in connection with her disappearance and suspected murder but was later released without charge, while a 47-year-old man remains on bail on suspicion of perverting the course of justice.
Mr Malyn said: "In the months since we started our review, we have made very significant progress.
"We have arrested and interviewed two men about Claudia's disappearance. One remains on bail while our inquiries in relation to him continue.
Miss Lawrence with her father Peter
"In the coming weeks and months we may make additional arrests as we continue to develop our lines of inquiry.
"In the light of information that has only recently come to the fore, we are actively pursuing what are rightly described as new leads."
Mr Malyn said he believes the alleyway has "potential significance" in explaining what happened to Miss Lawrence and is still being investigated.
The detective said some people locally knew Miss Lawrence but have kept their relationship secret and some have deliberately lied about a number of issues concerning their association with the chef, who was 35 at the time.
He said: "I would make a further appeal for those to come forward now and provide these explanations.
"If I reach the point where we believe people continue to obstruct this investigation or assist or cover up for the person or persons responsible, they will be arrested."
Mr Malyn continued: "Six years have passed since Claudia disappeared.
"For her loved ones, the pain caused by her absence and the circumstances of her disappearance has not faded. And nor, I assume, has the guilt and knowledge that one or more people have about what happened to this young woman. "
He added: "Ultimately, we may not be able to prove what happened to Claudia, or be able to find her. But that will not be for lack of determination, tenacity and meticulous detective work, nor whilst there are still clues that need to be investigated.
"If you know what happened to Claudia, or know that you have some relevant information but have not yet come forward, then do so, today.
"Six years is a long time to live with that knowledge."