An anonymous donor has offered a £20,000 reward to help find missing clubber Sian O'Callaghan.
Details of the offer came as hundreds of volunteers joined a search of woodland for the 22-year-old who has not been seen since the early hours of Saturday after leaving a nightclub.
Police issued a new poster, with details of the reward, which is being pinned up across Wiltshire by volunteers and also warned people to be extra careful and not to be out walking alone.
The poster includes a picture of Miss O'Callaghan and CCTV footage of her leaving the Suju nightclub in Swindon, Wiltshire, at 2.50am.
It reads: "A £20,000 reward has been offered anonymously for information that leads directly to Sian being found."
The poster, which features a picture of her mobile phone, also reveals that the LG E900 Optimus 7, was on in the 4,500-acre Savernake Forest area, near Marlborough, Wilts, at 3.24am.
It adds: "Please contact police immediately if you saw Sian, anyone with her, or any vehicles either after she left Suju or in or near beauty spots between Swindon and Savernake between 3am and 4am.
"Any information, no matter how trivial it may seem, could be vital to police enquiries."
Today around 400 people, including friends of Miss O'Callaghan, queued up to be split into teams to join specialist police officers combing the thick undergrowth of Savernake Forest, near Marlborough, Wilts, for clues to her disappearance.
Also assisting in the search were police dogs and members of the Wiltshire Search and Rescue team. Police had also drafted in other specialist search and rescue experts to help.
So many people turned out to assist the search effort that by mid-afternoon police had asked for no one else to come to the Postern Hill campsite, on main A346 Salisbury to Marlborough road, to help.
Miss O'Callaghan's boyfriend Kevin Reape, 25, is also believed to have joined the searches, which are carrying on until dusk.
Among those helping was Mikey Jack, 19, who plays Sunday league football with Mr Reape.
He said: "A lot of people here don't really know Kevin or Sian but they just want to help any way they can.
"Kevin's a great lad and he doesn't deserve this and my thoughts are with him and his family. He's a strong lad and I think deep down he's hurting a lot."
Another volunteer, college student Tizanne Gregory, 17, added: "I came here today in the hope of finding Sian.
"I'm just hoping we can do anything to help her parents and it is horrible not knowing where she is."
Miss O'Callaghan has not been since she left the Old Town area to walk the half-mile home to the flat she shared with her boyfriend.
More than 60 officers are working on the investigation, which they are treating as a missing person inquiry.
Detective Superintendent Steve Fulcher, who is leading the inquiry, said: "Firstly, I'd like to thank the public and media for their ongoing support with our investigation.
"My teams are working tirelessly to find Sian and the community have been very helpful and understanding.
"We know that Sian had an LG E900 Optimus mobile phone with her on the night of her disappearance and I'd like to hear from anyone who finds a phone of this description.
"Sian left Suju and walked along the High Street in Old Town and there were a number of vehicles moving through the High Street between 2.55am to 3am.
"We would like to identify the people in those vehicles as possible witnesses.
"I'd also like to hear from anyone who saw any vehicles in or near beauty spots between Swindon and Savernake between 3am and 4am.
"It is very important that people come forward as they may have vital information which will help us to find Sian.
"I'd also like to take this opportunity to remind people to think about their personal safety and take basic measures such as always making sure someone knows where you are going and what time you will be home, stay with friends and try not to walk alone."
Wiltshire Police have released footage of Miss O'Callaghan alone in the nightclub and hope it will jog revellers' memories about what she was doing and who was with her.
The footage shows her walking downstairs and going round a corner out of shot and into another room.
The black-and-white footage continues showing her walk through the club's entrance and outside, still by herself, past a small group of men.
Police stressed the times shown on the CCTV were approximately six minutes slow and urged anyone who remembered seeing her to contact them.
A tearful Mr Reape, a quantity surveyor, said yesterday: "We all want to know where Sian is and we want her home safe and well.
"Sian is a bubbly, lively person and is instantly liked by everyone she meets."
Mr Reape, who raised the alarm at 9.45am on Saturday when she had not returned to their flat in Westmorland Road, added: "Someone out there must have seen or known where she is and we just want them to come forward and contact the police.
"This is a terrible time for all of us and we are praying for Sian's safe return.
"If Sian is listening and doesn't want to contact us, I beg her to at least ring the police."
Miss O'Callaghan is described as white, 5ft 3in tall, of medium build with brown asymmetric hair and green eyes.
When last seen, she was wearing a grey dress, black bolero-style jacket, dark-coloured tights and flat boots.
She was carrying a dark handbag which had a beige flower attached.
Anyone with information is asked to contact Wiltshire Police on 0845 408 7000 or Crimestoppers 0800 555 111.