The plane-strain response of an unbounded elastic body containing a semi-infinite crack subjected to a pair of concentrated forces suddenly applied to the crack faces at some distance from the crack tip is determined. The forces act on opposite faces of the crack, in the plane of the crack, and in the same direction. An exact solution is obtained within the framework of linear elastodynamics using a fundamental solution obtained from dynamic elastic dislocation theory. If the loading is quasi-statically applied, then the stress intensity factor is zero. However, if the loads are suddenly applied, the stress intensity factor varies with time, and, for a short time, it takes on very large values. As time becomes large compared to the transit time of a Rayleigh wave from the load point to the crack tip, the stress intensity factor decays to zero. The same procedure may be applied for growing cracks subjected to the same type of loading.