Amid the serenity and solemnity of the National September 11 Memorial, the two sunken granite pools that designate the footprints of the absent World Trade Center towers have become a natural focal point, drawing visitors with artificial waterfalls that extend three stories down.

But for the New York Police Department, the pools also represent a focal point for an entirely different reason: the fear that people overwhelmed by grief may try to commit suicide there.

Police officials and grief experts share concerns that the memorial poses a unique risk because of its layout and its powerful relationship to the terrorist act of Sept. 11, 2001, and because those who lost loved ones that day may still have unresolved issues of loss.

The concern is as yet unrealized; there have been a million or so visitors to the memorial since it opened last September, and there have been no suicide attempts. Nonetheless, the police said a plan had been put in place.