Four police officers in New York have been reassigned from the World Trade Center site after tests showed elevated levels of mercury in their blood.

A Port Authority spokesperson says none of the four officers have shown signs of mercury poisoning. The source of the mercury has not been determined.

Early air and water samples taken at the site failed to turn up any detectable levels of mercury, and officials were unable to find any large source of mercury in or around the twin towers.

High levels of mercury can damage the brain, kidneys and lungs.