  Using the Karl G. Jansky Very Large Array (VLA), we have detected absorption
lines due to carbon-monoxide, CO(J=0-1), and the cyano radical, CN(N=0-1),
associated with radio galaxy B2 0902+34 at redshift z=3.4. The detection of
millimeter-band absorption observed 1.5 Gyr after the Big Bang facilitates
studying molecular clouds down to gas masses inaccessible to emission-line
observations. The CO absorption in B2 0902+34 has a peak optical depth of
$\tau$ $\ge$ 8.6% and consists of two components, one of which has the same
redshift as previously detected 21-cm absorption of neutral hydrogen (HI) gas.
Each CO component traces an integrated H$_2$ column density of N(H2) $\ge$
3x10$^{20}$ cm$^{-2}$. CN absorption is detected for both CO components, as
well as for a blueshifted component not detected in CO, with CO/CN line ratios
ranging from $\le$0.4 to 2.4. We discuss the scenario that the absorption
components originate from collections of small and dense molecular clouds that
are embedded in a region with more diffuse gas and high turbulence, possibly
within the influence of the central Active Galactic Nucleus or starburst
region. The degree of reddening in B2 0902+34, with a rest-frame color B-K ~
4.2, is lower than the very red colors (B-K > 6) found among other known
redshifted CO absorption systems at z<1. Nevertheless, when including also the
many non-detections from the literature, a potential correlation between the
absorption-line strength and B-K color is evident, giving weight to the
argument that the red colors of CO absorbers are due to a high dust content.
