In a terrifying turn of a events, investigators discovered a "digital bomb" (piece of destructive code) planted in the Nasdaq exchange by assumed elite Russian hackers. Four years on investigators have a better idea of the event timeline, exactly what happened, and theories on motivation- but hard answers haven't been found.
Even if the original hacker group had no intention of using the attack code on the Nasdaq (perhaps it was a proof of efficacy test), other hacker groups could have easily initiated the sequence, causing a financial disaster that didn't have any readily-identifiable culprit. The internet is a scary place. Possible takeaways? Keep your computer patched and maybe consider a cybersecurity company to invest in.
In-depth, comprehensive summary of both the hack and subsequent investigation can be found on Bloomberg: https://www.bloomberg.com/news/articles/2014-07-17/how-russian-hackers-stole-the-nasdaq
