The welfare of a missing 11-year-old leukemia patient who was taken from a Phoenix hospital by her mother the day before she was scheduled to be discharged is the prime concern for authorities, who fear a catheter in the girl's heart could become infected and endanger her life.

The patient, who is known as Emily, recently battled an infection and had her right arm amputated. Surveillance video captured Emily, a young boy, and Emily's mother, who police are only identifying as Norma, walking out of Phoenix Children's Hospital at 10:30 p.m. on Wednesday.

"In this particular case, we have again an 11-year-old girl who can't really decide for herself," said Sgt. Steve Martos of the Phoenix Police Department. "Her parents are now removing her from the hospital and putting her in danger by not providing that last bit of medical attention that she needs."

The search has been difficult for officers, Martos said, since Emily and her family are from Mexico and have no locally listed records in Arizona.

A nurse supervisor called 911 when she realized Emily was missing and described how she was able to avoid detection.

"She was wearing a wig, which is not unusual , a lot of our cancer patients wear wigs," the supervisor said. "She wasn't wearing a wig when she went into the bathroom though and then she was wearing a wig when she came out and she was actually covering her right arm, the amputated arm."

Police said Norma removed Emily's IV before walking her out of the hospital in street clothes.

The family left the hospital in a black van and have not been seen since.

It's not clear how long Emily had been a patient at the hospital or what may have prompted her mother to take her before she was scheduled to be discharged.