In January, a 19 year-old man stormed into a skateboard shop in a Columbia mall with a shotgun and a backpack full of explosives. He fatally shot two people and injured a third before killing himself — and leaving many questions behind. Was it a case of random rage? Or a love triangle gone bad? After weeks of investigation, we may finally have some answers.

According to police, shooter Darion Marcus Aguilar knew he had a problem for months before the shooting. He told his doctor he heard voices, but never made a follow-up appointment with a psychiatrist. A year before the shooting, computer records show that he spent time online researching mental illness and suicide prevention–but he also scoured the internet for information about mass shootings, gun laws, and explosive techniques. The two people he killed were apparently strangers to him.

Police also revealed that Aguilar seemed to be obsessed with the Columbine school shootings, going so far as to take a photo of himself dressed up like the Columbine shooters, which he posted to Tumblr right before going on his own shooting rampage. The caption? “I had to do this.” (Police have declined to release the photo, claiming that it just plays into Aguilar’s desire for notoriety.) Entries from Aguilar’s journal told much the same story: He was a young man suffering from mental illness that inspired fantasies of violence and destruction that he managed to hide from everyone close to him.

“Nobody saw this coming,” Howard County Police Chief Bill McMahon told CNN. “He was a young man that didn’t garner much attention to himself.”