Michelle Hoang Thi Le, 26, vanished at around 7pm on Friday from Kaiser Permanente Medical Center in Hayward, California.

Witnesses said that she took a break from her clinical rounds and took her car keys and cell phone with her.

Speaking to KTVU.com, Miss Le's brother Michael said: 'It’s really hard not to assume the worst, but we’re trying.

'That’s the hardest thing, staying positive.'

Her car was found a few blocks from the hospital on Saturday with valuables still inside, but police have been unable to contact her on her phone.

The cell phone signal was traced to Oakland, Union City and Fremont, but can no longer be picked up.

‘The reality is, it's a pretty large area,’ Roger Keener of Hayward police told KGO.

‘We've had the helicopter fly over the area. We haven't found anything at all that leads us [in] one direction or another, that tells us this is a foul play incident or anything.’

Officers said they have examined surveillance video from the hospital car park, but as yet they have found no sign of Miss Le.

Miss Le had reportedly told some classmates that she had plans to go to Reno, Nevada, on Friday night.

Police said that she does not have any medical or psychiatric conditions that could account for her disappearance.

Miss Le was a student at Samuel Merritt University in Oakland, California.

‘The entire Samuel Merritt University community is deeply concerned about the disappearance of our nursing student Michelle Le,’ a university spokesman told ABC News.

‘She was last seen Friday evening after she did not return to the hospital from the parking garage at the Kaiser Permanente Hayward Medical Center.’ Source