  The KArlsruhe TRItium Neutrino (KATRIN) experiment aims to make a
model-independent determination of the effective electron antineutrino mass
with a sensitivity of 0.2 eV/c$^{2}$. It investigates the kinematics of
$\beta$-particles from tritium $\beta$-decay close to the endpoint of the
energy spectrum. Because the KATRIN main spectrometer (MS) is located above
ground, muon-induced backgrounds are of particular concern. Coincidence
measurements with the MS and a scintillator-based muon detector system
confirmed the model of secondary electron production by cosmic-ray muons inside
the MS. Correlation measurements with the same setup showed that about $12\%$
of secondary electrons emitted from the inner surface are induced by cosmic-ray
muons, with approximately one secondary electron produced for every 17 muon
crossings. However, the magnetic and electrostatic shielding of the MS is able
to efficiently suppress these electrons, and we find that muons are responsible
for less than $17\%$ ($90\%$ confidence level) of the overall MS background.
