  It has been suggested that the well-studied giant HII regions M16 and M17 may
have had a common origin, being an example of large-scale triggered star
formation. While some features of the distribution of the interstellar medium
in the region support this interpretation, no definitive detection of an
earlier population of massive stars responsible for the triggering has been
made thus far. We have carried out observations looking for red supergiants in
the area covered by a giant shell seen in HI and CO centered on galactic
coordinates $l \sim 14^\circ 5$, $b\sim +1^\circ$ that peaks near the same
radial velocity as the bulk of the emission from both giant HII regions, which
are located along the shell. Red supergiants have ages in the range expected
for the parent association whose most massive members could have triggered the
formation of the shell and of the giant HII regions along its rim. Out of a
sample of 37 bright red stars, we identify four red supergiants that confirm
the existence of massive stars in the age range between $\sim 10$ and $\sim
30$~Myr in the area. At least three of them have Gaia DR2 parallaxes consistent
with them being at the same distance as M16 and M17. The evidence of past
massive star formation within the area of the gaseous shell lends support to
the idea that it was formed by the combined action of stellar winds and
ionizing radiation of the precursors of the current red supergiants. These
could be the remnants of a richer population, whose most massive members have
exploded already as core-collapse supernovae. The expansion of the shell
against the surrounding medium, perhaps combined with the overrun of
preexisting clouds, is thus a plausible trigger of the formation of a second
generation of stars currently responsible for the ionization of M16 and M17.
