The Nashville Christmas bomber shared bizarre conspiracy theories about lizards controlling the world and aliens shortly before the attack.
Anthony Warner, 63, also rambled on about 9/11 and the moon landings in a letter to a friend.
The suicide bomber died and three people were injured in the Christmas Day blast last year in the Tennessee city.
More than 30 businesses were also damaged from the explosion in the heart of America’s country music capital, the Mirror Online reports.
Days later, on New Year's Day, a man who had known Warner received a mystery package containing nine typed pages and two Samsung thumb drives.
The note, postmarked December 23, was signed by "Julio" - a name Warner's pals say he often used when sending them emails.
A source told NewsChannel 5 Investigates Warner had a dog named Julio.
The letter starts: "Hey Dude, You will never believe what I found in the park.
"The knowledge I have gained is immeasurable. I now understand everything, and I mean everything from who/what we really are, to what the known universe really is."
Among the disturbing contents, he wrote about 9/11 conspiracy theories.
He said: "The moon landing and 9-11 have so many anomalies they are hard to count."
Warner also claimed "September 2011 was supposed to be the end game for the planet”.
He claimed it was because "that is when he believed that aliens and UFO's began launching attacks on earth".
He said the media was covering up those attacks.
And he claimed reptilians and lizard people controlled the earth and had tweaked human DNA.
He said: "They put a switch into the human brain so they could walk among us and appear human."
An AT&T building was destroyed in the blast by Warner, causing internet outages in four states.
Conspiracy theories baselessly connecting the 5G mobile network to the coronavirus pandemic have previously led to attacks on mobile masts worldwide.
FBI agents have reportedly quizzed Warner’s former colleagues and neighbours about whether he had a fixation on 5G technology.
In the letter Warner never mentioned AT&T or anything else that could suggest why he blew himself up.
But he did say "everything is an illusion" and "there is no such thing as death” in the note.
The letter urged the man to watch internet videos he had included on the two Samsung thumb drives.
The friend handed over the package and all its contents to the FBI.
Warner also sent packages to other friends, it has been claimed.
His home in Antioch, southeast of Nashville, was searched by the FBI and the US Bureau of Alcohol, Tobacco, Firearms and Explosives after the bombing.
The motorhome he was driving broadcast a message warning a bomb was going to explode shortly before the blast.
Heroic police officers rapidly evacuated residents from the area, and were credited for saving lives as the explosion ripped through the downtown area.