The fbi spokesman said that they “have strong evidence that suggests it was the subject that wrote it, but that they had not conclusively proven it yet”. Not kremlin talking points. Right out of the mouth of the fbi rep at the podium.
Here is a cbs article that gets at the content
https://www.cbsnews.com/news/las-vegas-cybertruck-bombing-matthew-livelsberger-letter-mental-health-crisis/
A review of Livensberger's social media posts, interviews with friends who knew him well, and most pointedly, his email to Shoemate, offered indications he may have been struggling with those issues. On Friday, Shoemate discussed the letter on The Shawn Ryan Show, a podcast where Ryan, a former Navy SEAL and CIA contractor, interviews individuals primarily from the national security space.
In the email obtained by CBS News, Livensberger referenced his ongoing concerns about civilian casualties caused by an airstrike in Afghanistan. The incident he mentioned appears to align with reports of U.S. airstrikes in May 2019 targeting alleged drug-processing facilities in Afghanistan, where more than 30 civilians, including children, were reportedly killed.