Jakubowski's stepfather, Don McClean, also cautioned both citizens and police against approaching his stepson.
"I don't want anyone hurt," McClean told ABC News. "I wouldn't approach him and I would tell police to approach him with extreme caution. He had always hated police, even as a kid."
Authorities said they are anticipating "some kind of endgame" with Joseph Jakubowski, who is considered "armed and dangerous" and sent the president a manifesto littered with anti-religious and anti-government rhetoric.
Janesville Police Chief David Moore and Rock County Sheriff Robert Spoden spoke to "GMA" this morning about Jakubowski, who police say allegedly broke into the Armageddon Supplies gun shop in Janesville, Wisconsin, on the night of April 4 and stole about 16 high-end firearms, likely with the intention of carrying out some sort of revolutionary plot.
"We think there's some kind of endgame," Spoden said in reference to a potential confrontation with the law enforcement community.
McClean said that he and his wife have been estranged from Jakubowski for two years. McClean said he'd never heard his stepson talk "pro or con" when it came to God or church.
"But, I can tell you my wife is doing a lot of crying and praying," he said. "She is very upset. We just want this to be over."
Jakubowski's cousin, who wished to remain anonymous, told ABC News that he had coincidentally run into his cousin a couple weeks ago when he was making a photo copy of the manifesto.
When Jakubowski asked his cousin to look at it, he said he told him, "I'm not putting my fingerprints on that."
Jakubowski's cousin speculates that the document he drafted is not a manifesto, but rather a "letter to vent." He also scoffed at the notion that his cousin is currently hiding out in the woods, saying he is probably in a larger city -- like Madison, Wisconsin -- where he could "blend in."
The owner of Armageddon Supplies, Scott Kuhn told ABC News, when asked if Jakubowski was a regular customer, that "This is the first I've heard of this."
The 161-page handwritten manifesto Jakubowski mailed to Trump has already reached the White House, according to police, and its content has concerned investigators.
"With regard to the manifesto," Moore said, "two things concern us. One is anti-religion and one is anti-government [rhetoric]."
A Facebook video recorded by Jakubowski a week before the robbery also provides hints of his possible intentions.
"Revolution," Jakubowski says into the camera in the video. "It's time for change."
Spoden noted that Jakubowski had run-ins with the police prior to disappearing, including one incident in which he tried to disarm a police officer. But this most recent issue is most concerning because of the type of language he has used to describe his potential ambitions.
"He's had a long history of run-ins with local police," Spoden said. "He kind of went in a totally different direction and started talking about revolution and things such as that."
Police have also released pictures of Jakubowski's tattoos in the hope that will help people identify him.
Moore said the police have received 542 leads so far in their investigation.
Over the weekend, police suggested that Jakubowski may have left Wisconsin and noted that federal officials are looking beyond the state for him.
This Sunday, police monitored churches out of concern that he might make a religious service the target of an attack.