  Both theoretical and observational results suggest that high-redshift radio
galaxies (HzRGs) inhabit overdense regions of the universe and might be the
progenitors of local, massive galaxies residing in the centre of galaxy
clusters. In this paper we present CO(3-2) line observations of the HzRG TXS
0828+193 (z=2.57) and its environment using the Atacama Large
Millimeter/submillimeter Array. In contrast to previous observations, we detect
CO emission associated with the HzRG and derive a molecular gas mass of
$(0.9\pm0.3)\times10^{10}\,\rm M_{\odot}$. Moreover, we confirm the presence of
a previously detected off-source CO emitting region (companion #1), and detect
three new potential companions. The molecular gas mass of each companion is
comparable to that of the HzRG. Companion #1 is aligned with the axis of the
radio jet and has stellar emission detected by Spitzer. Thus this source might
be a normal star-forming galaxy or alternatively a result of jet-induced star
formation. The newly found CO sources do not have counterparts in any other
observing band and could be high-density clouds in the halo of TXS 0828+193 and
thus potentially linked to the large-scale filamentary structure of the cosmic
web.
