Four men arrested last year on suspicion of murdering chef Claudia Lawrence will not face charges, police have confirmed.
The men were arrested in March and April 2015, shortly after a renewed appeal by North Yorkshire Police on the sixth anniversary of Miss Lawrence's disappearance.
The 35-year-old was last seen on March 18 2009 and was reported missing the following day when she failed to turn up for a shift at work at the University of York.
North Yorkshire Police confirmed the Crown Prosecution Service decision not to charge the four men with any offences.
Detective Superintendent Dai Malyn said: "For legal reasons I am limited about what I can say, but I respect the decision made by the Crown Prosecution Service.
"We have made several arrests in this inquiry and, as a result, we have had positive reactions from the public and very useful information.
"We do not arrest people without weighing up the proportionality and justification for the arrest.
"To eliminate people from an inquiry, we sometimes have to search for evidence that either supports their alibis or not. Seven years on this is more difficult to do just relying on memory, therefore if it is justified, proportionate and necessary, we will arrest or apply for search warrants.
"The investigation team will continue to thoroughly assess any new leads and information. We remind those who we suspect are withholding key information about Claudia that we will not give up our quest to find the truth."
Martin Dales, friend and spokesman of Miss Lawrence's father Peter, described the unsolved investigation as "depressing".
He said: "I know Peter, the family and Claudia's friends will be feeling that the search for what has happened to Claudia seems to have turned out to be yet another false dawn in an already forlorn situation.
"The police have worked tirelessly to try and find answers but currently without success.
"It is nearly the seventh anniversary of Claudia disappearing and it seems that we're all back to square one: it's really quite depressing especially as someone is clearly withholding the information needed."
Mr Malyn began a review of the original North Yorkshire Police investigation in the autumn of 2013.
Up to 20 detectives and police staff have worked full-time on the review, with the support of many others brought in during the last two-and-a-half years, at a cost of nearly £800,000.
On the sixth anniversary of Miss Lawrence's disappearance last year, detectives released previously unseen CCTV footage of a man acting suspiciously close to the chef's house in Heworth, York.
The CCTV footage showed a man in Lime Court, Heworth Road, on the morning after Miss Lawrence disappeared. Additional footage from the same camera appears to show the same man in the same place the previous evening.
Following the release of the footage, the four men, in their 50s and from the York area, were arrested on March 23 and April 22 on suspicion of murder.
They were released from bail in September to allow time for the CPS to assess the file of evidence and consider whether to bring charges.
A 59-year-old man was arrested on suspicion of murder in May 2014 and was later released from bail and a 46-year-old man was arrested in July 2014 on suspicion of perverting the course of justice. He was later released without charge.
Mr Malyn said the investigation has been compromised by the reluctance and refusal of some people to co-operate with police inquiries.
He said he is now sure there are a number of people who know what happened to Miss Lawrence but who have refused to come forward.
And the detective believes the man pictured on CCTV at the time of her disappearance "remains the most important person to trace".
Mr Malyn said: "I am sure that there are some people who know, or who have very strong suspicions about, what happened to Claudia. For whatever reason, they have either refused to come forward, or have been economic with the truth.
"I am left with the inescapable conclusion that this case could still be solved if only people were honest with us. The fact that they are not is agonising for Claudia's family and they should be ashamed of themselves."
He added: "As we move towards the seventh anniversary of Claudia's disappearance, we hope the public will continue to help us and provide that vital information the family and we need to find out what happened to Claudia."
Police said they will "never give up on Claudia".
Assistant Chief Constable Paul Kennedy said: "This investigation will never be closed before we find the truth. If new information comes to light about Claudia Lawrence or anyone involved in her disappearance and suspected murder, North Yorkshire Police will act immediately."