  The \MJ\ Collaboration is operating an array of high purity Ge detectors to
search for neutrinoless double-beta decay in $^{76}$Ge. The \MJ\ \DEM\
comprises 44.1~kg of Ge detectors (29.7 kg enriched in $^{76}$Ge) split between
two modules contained in a low background shield at the Sanford Underground
Research Facility in Lead, South Dakota. Here we present results from data
taken during construction, commissioning, and the start of full operations. We
achieve unprecedented energy resolution of 2.5 keV FWHM at \qval\ and a very
low background with no observed candidate events in 10 kg yr of enriched Ge
exposure, resulting in a lower limit on the half-life of $1.9\times10^{25}$ yr
(90\% CL). This result constrains the effective Majorana neutrino mass to below
240 to 520 meV, depending on the matrix elements used. In our experimental
configuration with the lowest background, the background is $4.0_{-2.5}^{+3.1}$
counts/(FWHM t yr).
