In a nutshell, they basically recalculated the entry trajectory of what ever it was the collided with the earth. Based on this, they dug up some new areas and found some metalic/unknown material composite fragment. So they've claimed to have an "alien mechanical device".
They'd found metallic fragments in the past, but these turned out to be industrial refuse. This fragment apparently is a lot more suspicious.
the team that made the discovery, apparently went out there intending to find alien stuff.