He allegedly posted his intentions on 4Chan.* He said "I'm going to kill myself tomorrow and it's going to make the news. Be watching at 9am." Under that he posted the picture of Dylan Klebold and Eric Harris dead in Columbine High School. Someone then asked where he lived, and he said Connecticut.
*DO NOT GO TO 4CHAN! It's the sewer of the internet. Dead/tortured animals, pornography, snuff photos etc. Grisly and disturbing.
While the gunman is thought to have acted alone, the law enforcement source said a deep dive into Lanza’s computers could provide more clues.
“You don’t know if this kid was put up to this by somebody else,” the source said. “You don’t know if there was a conspiracy of sorts. You don’t know if there wasn’t somebody who wasn’t goading this kid on.”