The investigation into the disappearance of chef Claudia Lawrence has been compromised by people refusing to help police, detectives have said.
Detective Superintendent Dai Malyn of North Yorkshire Police was speaking after a Crown Prosecution Service decision not to bring charges against four men arrested last year on suspicion of Miss Lawrence's murder.
Miss Lawrence's family said it seemed they were "back to square one" following the latest "depressing" development in the seven-year inquiry.
Police confirmed on Tuesday evening that the four men, in their 50s and from the York area, would not be charged with any offences following their arrests in March and April 2015.
Miss Lawrence was last seen on March 18 2009 and was reported missing the following day after she failed to turn up for a shift at work at the University of York.
Police have said they believe the 35-year-old was murdered, although no body has ever been discovered.
A review of the original investigation began in the autumn of 2013 and has resulted in six arrests. No charges have been brought.
Mr Malyn said the investigation had been compromised by the reluctance and refusal of some people to co-operate with police inquiries.
He 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: "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, said it was clear that someone was withholding information.
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."
Since the beginning of the review, which has cost nearly £800,000, detectives have released CCTV footage of a man acting suspiciously in Lime Court, Heworth Road, close to the chef's house, around the time of her disappearance.
Mr Malyn said he believes the man pictured on CCTV "remains the most important person to trace" and added that police 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."
Anyone with information that could assist the investigation should contact North Yorkshire Police on 101 and quote Claudia Lawrence when passing on details. Or contact the independent charity Crimestoppers anonymously on 0800 555111 or at www.crimestoppers-uk.org.