Police arrested a Mount Vernon woman they say admitted calling in one of two bomb threats to Westchester Medical Center in Valhalla Wednesday.

Police say the first threat was called into the hospital switchboard at 8:18 a.m. by a man. Police and bomb-sniffing dogs combed the buildings, but found nothing. Police rushed back into the hospital when a second threat of explosives planted somewhere in the complex came in at about 10:30 a.m. Another search of the hospital turned up nothing. Police arrested 24-year-old Kadian Sewell around 2 p.m. They say she made the second call from a public telephone in one of the hospital buildings.

Sewell is charged with aggravated harassment, a threat by use of a telephone and illegally reporting an incident. Police say Sewell has a prior history of making threats and was convicted in 2002 of calling in a bomb threat to Mount Vernon Police Headquarters. Police are still trying to trace the first threat.