  Topological insulators (TIs) constitute a new class of materials with an
energy gap in the bulk and peculiar metallic states on the surface. To date,
most experiments have focused on probing the surface electronic structure of
these materials. Here, we report on new and potentially interesting features
resulting from the bulk electronic structure. Our findings are based on a
comprehensive nuclear magnetic resonance (NMR) study of $^{77}$Se on
Bi$_2$Se$_3$ and Cu$_{0.15}$Bi$_2$Se$_3$ single crystals. First, we find two
resonance lines and show that they originate from the two inequivalent Se
lattice sites. Second, we observe unusual field-independent linewidths, and
attribute them to an unexpectedly strong internuclear coupling mediated by bulk
electrons. These results call for a revision of earlier NMR studies and add
insight into the bulk electronic properties of TIs.
