  We study 6j-symbols, or Racah coefficients for tensor products of
infinite-dimensional unitary principal series representations of the group
SL(2,C). These symbols were constructed earlier by Ismagilov and we rederive
his result (up to some slight difference associated with equivalent
representations) using the Feynman diagrams technique. The resulting 6j-symbols
are expressed either as a triple integral over complex plane with a symmetric
kernel or as an infinite bilateral sum of integrals of the Mellin-Barnes type.
