  GW190521 challenges our understanding of the late-stage evolution of massive
stars and the effects of the pair-instability in particular. We discuss the
possibility that stars at low or zero metallicity could retain most of their
hydrogen envelope until the pre-supernova stage, avoid the pulsational
pair-instability regime and produce a black hole with a mass in the mass gap by
fallback. We present a series of new stellar evolution models at zero and low
metallicity computed with the Geneva and MESA stellar evolution codes and
compare to existing grids of models. Models with a metallicity in the range
0-0.0004 have three properties which favour higher BH masses as compared to
higher metallicity models. These are (i) lower mass-loss rates during the
post-MS phase, (ii) a more compact star disfavouring binary interaction and
(iii) possible H-He shell interactions which lower the CO core mass. We
conclude that it is possible that GW190521 may be the merger of black holes
produced directly by massive stars from the first stellar generations. Our
models indicate BH masses up to 70-75 Msun. Uncertainties related to convective
mixing, mass loss, H-He shell interactions and pair-instability pulsations may
increase this limit to ~85 Msun.
