(CNN) New Zealand police said they believe missing British backpacker Grace Millane is dead, and a man seen with her in Auckland the night she disappeared will be charged with her murder.

Grace Millane, 22, was last seen at the Auckland city center around 7 pm December 1. For days, local police searched for her, but new evidence changed the course from a missing person investigation to a murder case.

"The evidence we have located so far -- our scene examination, CCTV footage and our investigation has determined that Grace is no longer alive and this is a murder investigation," Auckland Police Detective Scott Beard said at a news conference Saturday.

Police said they detained a 26-year-old man seen with her on the night of her disappearance, and will charge her with Millane's murder Monday. They believe he and Grace visited a number of locations that evening, and are looking for information on a vehicle linked to the case.

Millane had been in New Zealand for two weeks, the UK's Press Association news agency reported.

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