  Hypermassive neutron stars (HMNSs) -- equilibrium configurations supported
against collapse by rapid differential rotation -- are possible transient
remnants of binary neutron star mergers. Using newly developed codes for
magnetohydrodynamic simulations in dynamical spacetimes, we are able to track
the evolution of a magnetized HMNS in full general relativity for the first
time. We find that secular angular momentum transport due to magnetic braking
and the magnetorotational instability results in the collapse of an HMNS to a
rotating black hole, accompanied by a gravitational wave burst. The nascent
black hole is surrounded by a hot, massive torus undergoing quasistationary
accretion and a collimated magnetic field. This scenario suggests that HMNS
collapse is a possible candidate for the central engine of short gamma-ray
bursts.
