Officials say there were few red flags from Dimitrios Pagourtzis, the 17-year-old facing capital murder charges over Friday's Santa Fe High School shooting in Texas.
"His slate is pretty clean. There simply were not the same type of warning signs that we've seen in so many other shootings."
However, hours before he allegedly stormed into an art class armed with a shotgun and revolver the teenager made a weird post on social media, a law enforcement source told CBS News.
Accompanied by an occult symbol, it said simply, "Dangerous Days".
He had also previously posted an image of a trench coat pinned with various insignia, including the Iron Cross used by the Nazis, which the teen wrote represented "bravery".
The Communist hammer and sickle pin, he said, stood for "rebellion", and a depiction of the idol Baphomet symbolised "Evil".
Police say the teenager detailed his plans to carry out the school shooting in a diary, on his computer and on a mobile phone.
The suspect had planned to take his own life, say investigators, but he ultimately gave himself up
