Police are questioning six Cambodian men about the disappearance of British backpacker Amelia Bambridge.
The 21-year-old from Worthing, West Sussex, was last seen on the island of Koh Rong late on Wednesday night.
Noun Bunthol, the governor of Koh Rong where she was last seen, said no charges had been filed against the six, who work at restaurants, hotels and guesthouses on the island.
He said police received a note from some Western visitors saying the men had acted “badly” toward foreign tourists in the past, especially females.
Cambodian police official Chuon Narin said: “We have decided the search mission will not be ended until we find her alive, or her body if she has died.”
Staff at Police Beach, a private venue that stages regular events, found Bambridge’s purple rucksack, containing her purse, phone and bank cards, the following morning.
About 150 volunteers and Cambodian police are involved in searches of the shoreline and jungle for the missing woman.
Her father Phil Bambridge, who has travelled to the island, said he believed she had been “taken”.
He told Sky News: “I don’t think she’s had an accident. If she’d had an accident she would have been found by now.”
Bambridge said that, having viewed CCTV, he believed she had been lost inland.
He said: “So we sort of knew from this here we were getting to fixed times and where she got there. I don’t think she’s in the sea, I think she’s inland somewhere.”
During a phone call with her younger sister Georgie on Wednesday, Bambridge said her first solo trip was “doing so much for her confidence” and she was having “the best time ever”.
Georgie Bambridge told Sky News: “She said ‘everyone is so cool here’ and couldn’t be happier.
“She’s literally wanted to go travelling her whole life.
“She has spent the past two years saving for it and spent the last year reading blogs of travellers, researching what she was going to do.
“She knew everything and was living out her dream – she was happy.”
Bambridge said her “role model” sister “would never drink to excess” and was “always in control”, adding: “She would always be looking out for me, it’s not like her to not look after herself.
“I just don’t know what’s happened.”
Bambridge set off on her trip on September 27, when she flew to Vietnam, her family said.
They were alerted to her disappearance after she failed to check out of the Nest Beach Club hostel after she was last seen at a beach party.
Her passport remained at the hostel.
Major general Chuon Narin, police chief of coastal Preah Sihanouk province, said divers, police, navy personnel, local volunteers and foreigners were taking part in the searches.
He told AP: “As of now, the searchers have no news about her whereabouts and they have expanded the area they are searching to make sure all possible sites are checked.
“I can’t say whether Bambridge was killed or got lost in the island’s jungle.”
He added that several foreign students have previously been found safe after going missing in a jungle area and said he wasn’t aware of any cases of foreigners being murdered.
However one local official, Koh Rong commune chief Chhoeun Chantha, told The Associated Press he suspected Bambridge may have drowned because the authorities found her belongings on a rock close the water’s edge near the party site.
Bambridge’s brother Harry wrote on Facebook on Sunday: “So I’ve landed in Cambodia, in a taxi to attempt to make the last ferry to the island, which is a five hour taxi.
“I’m totally overwhelmed with everyone’s support and I can’t thank you all enough.”
In a different post, he added: “I seriously believe there is still hope for her to be found alive.”