  Magnetars are young neutron stars with extreme magnetic fields (B >
10^{14}-10^{15}G). How these fields relate to the properties of their
progenitor stars is not yet clearly established. However, from the few objects
associated with young clusters it has been possible to estimate the initial
masses of the progenitors, with results indicating that a very massive
progenitor star (M_prog >40Msun) is required to produce a magnetar. Here we
present adaptive-optics assisted Keck/NIRC2 imaging and Keck/NIRSPEC
spectroscopy of the cluster associated with the magnetar SGR 1900+14, and
report that the initial progenitor star mass of the magnetar was a factor of
two lower than this limit, M_prog=17 \pm 2 Msun. Our result presents a strong
challenge to the concept that magnetars can only result from very massive
progenitors. Instead, we favour a mechanism which is dependent on more than
just initial stellar mass for the production of these extreme magnetic fields,
such as the "fossil-field" model or a process involving close binary evolution.
