The all clear has been given on Capitol Hill in Washington after a major security alert. Some 200 members of staff were evacuated from a US Senate office buliding after detectors indicated the presence of a possible nerve agent. Several senators were among those forced to leave. A police hazardous materials unit was called in but tests proved negative after a scare lasting three hours. It is believed substances as benign as perfume, cleaning fluid or fertilizer could have been responsible for the false alarm at the multi-storey Russell Office building.