It's unclear if Russia was directly responsible for the chaos during the opening ceremony, which disrupted internet and broadcast systems to the point where some guests couldn't even print their tickets. However, the new evidence suggests Russia was in a prime position for such an attack. It also has the motivation: the country hasn't been shy about voicing its anger over its ban following revelations of state-organized doping, and it leaked Olympic athletes' health data following the 2016 Summer Olympics. North Korea is considered an unlikely candidate as it was using the games to improve relations with South Korea.
The same unit responsible for these hacks is also believed to be responsible for the NotPetya attacks that wiped computers in numerous countries during 2017.
Neither US officials nor Olympics overseers have publicly blamed Russia, and it's not certain they ever will. Olympics representatives hinted that they knew who perpetrated the opening ceremony attack but were keeping it a secret. And it's easy to see reasons why they wouldn't want to lay blame even now. Organizers risked stoking tensions at the same time as they were running an event promoting world harmony. It's virtually certain that Russia would deny any involvement, too, even if there was smoking gun evidence. As it stands, the hacking campaign suggests that future games' overseers will have to treat cybersecurity as a top priority.