While Bergen County prosecutor John Molinelli did not call it a suicide note, he said it did "express that an end is coming. It could have been prison. ... It could have been what he did last night. It gave his family reason to reach out to us."
Investigators don't believe Shoop was intent on shooting people. They say he began shooting randomly in an open area of the mall, but did not take aim at anyone.
"Based upon what we know, he did not fire the weapon under circumstances that one would think he would think he would be looking to kill someone, because there were a lot of people around him, witnesses within a matter of feet," said Molinelli.
Kevin Shoop, the suspect's brother, told reporters outside their home in Teaneck that he had no advance warning about what his brother was going to do.
"He just sadly decided to make an act of - an act of, I guess, self-indulgence - by taking his own life publicly," Kevin Shoop said. "And it's a tragedy to us all. And we're going to now handle matters and deal with them."
Kevin Shoop said he is devastated, and called his brother a great person who was liked by his friends and family.
Kevin Shoop said his brother used a weapon that did not belong to him.
"As far as anything goes into the drugs or any of the other rumors out there, no comment," Kevin Shoop said.
Authorities say Shoop entered the upscale mall around 9:20 p.m. wearing all black, a motorcycle helmet and carrying a large rifle.
After the gunman fired at least six shots, authorities lost him in the mall. The SWAT team and K9 units went store to store searching for him. Six hours later, authorities saw the suspect on surveillance video go to a secluded part of the mall and kill himself.
Police say Shoop did not work at the mall, and they are still trying to determine why he went there.
Police say Shoop was a known drug user.
The Associated Press contributed to this report.