Case 1:
Let your linguists start working on their language (Initiate Communication on the CTRL-F5 window)
Send a cheap scout and pray it gathers more intel before being blown to smithereens
You can also try to drop an espionage team onto their planet.
Destroying their ships and rescueing their life pods also can give you more intel
Destroying their ships and salvaging the wrecks will give you some intel on what those ships mounted (beware tho, as those ships might not have been their state-of-the-art ones).
Case 2:
If no new race shows up, there are two possiblities:
1. It is a race you already know, i.e. the race from case 1
2. Your ship was not destroyed by an alien race at all but run into some leftover mines.
P.S.
You got 18 contacts in case 1, why do you assume those mesons that killed your ship were planet-based? I wouldn´t be so sure about that - I won´t say any more.
Have fun working out what is going on