  Optically very faint R>25.5 sources detected by the Spitzer Space Telescope
at 24um represent a very interesting population at redshift z [1.5-3]. They
exhibit strong clustering properties, implying that they are hosted by very
massive halos, and their mid-IR emission could be powered by either
dust-enshrouded star-formation and/or by an obscured AGN. We report
observations carried out with the MAMBO array at the IRAM 30m antenna on Pico
Veleta of a candidate protocluster with five optically obscured sources
selected from the 24um Spitzer sample of the First Look Survey. Interestingly,
these sources appear to lie on a high density filament aligned with the two
radio jets of an AGN. Four out of five of the observed sources were detected.
We combine these measurements with optical, infrared and radio observations to
probe}the nature of the candidate protocluster members. Our preliminary
conclusions can be summarized as follows: the Spectral Energy Distributions of
all sources include both AGN and starburst contributions; the AGN contribution
to the bolometric luminosities ranges between 14 and 26% of the total. Such a
contribution is enough for the AGN to dominate the emission at 5.8, 8 and 24um,
while the stellar component, inferred from SED fitting, prevails at 1.25mm and
at lambda<4.5um. The present analysis suggests a coherent interplay at high-z
between extended radio activity and the development of filamentary large-scale
structures.
