  Our search for the earliest stages of massive star formation turned up twelve
massive pre-protocluster candidates plus a few protoclusters. For this search,
we selected 47 FIR-bright IRAS sources in the outer Galaxy. We mapped regions
of several square arcminutes around the IRAS source in the millimeter continuum
in order to find massive cold cloud cores possibly being in a very early stage
of massive star formation. Masses and densities are derived for the 128
molecular cloud cores found in the obtained maps. We present these maps
together with near-infrared, mid-infrared, and radio data collected from the
2MASS, MSX, and NVSS catalogs. Further data from the literature on detections
of high-density tracers, outflows, and masers are added. The multi-wavelength
datasets are used to characterize each observed region. The massive cloud cores
(M>100 M_sun) are placed in a tentative evolutionary sequence depending on
their emission at the investigated wavelengths. Candidates for the youngest
stages of massive star formation are identified by the lack of detections in
the above-mentioned near-infrared, mid-infrared, and radio surveys. Twelve
massive cores prominent in the millimeter continuum fulfill this requirement.
Since neither FIR nor radio emission have been detected from these cloud cores
massive protostars must be very deeply embedded in these cores. Some of these
objects may actually Pre-Proto-cluster cores: an up to now rare object class,
where the initial conditions of massive star formation can be studied.
