Motive still unclear in US hospital attack that left one police officer dead

A woman leaves flowers in front of the West York Police Department after an officer was killed responding to a shooting at UPMC Memorial Hospita (Matt Rourke/AP)

Investigators are trying to determine why an armed man targeted a hospital in the US state of Pennsylvania, killing one officer and injuring several medical staffers.

Bearing a pistol, the armed man entered the UPMC Memorial Hospital intensive care unit (ICU) on Saturday and took staff hostage prior to his death, with officials still unsure of the reason for the violent attack.