Hours earlier, Lawhorn, 23, had told dispatchers that he was going to a remote location, north of town, to commit suicide, said Stan Allen, assistant police chief in East Ridge, Tennessee.
But Lawhorn appears to have traveled in the opposite direction.
He was seen getting petrol in Dade County, Georgia, a few kilometres south of the Tennessee border, a half-hour after he made the emergency call.
He then drove about 480km southwest across Alabama and into Mississippi, authorities said.
At some point yesterday, after hours of driving, Lawhorn reportedly wrote an ominous Facebook post about the killings.
"As I sit here in Mississippi, writing this on the side of I-59 south after my car broke down, what I look forward to is the nothingness after death," he wrote, the Clarion-Ledger reported. "I've spent a lot of time thinking about murder, wondering what it feels like. But I've barely felt anything."
Allen, the assistant police chief, said the now-deleted post appeared to be a confession to the two killings, which Lawhorn described at length and in detail.
"Since so many people keep trying to reach out to me on Facebook, I figure I can just say my piece once here and be done with it," Lawhorn wrote, according to the Clarion-Ledger.
"Before I start, I need to emphasise that I take full responsibility for my actions. Nothing anyone has or hasn't done to me caused this, my decisions and my failures are my own."
He went on to say that he killed his mother, Vi Lawhorn, and his close friend who was staying in the East Ridge home with a stolen gun.
The friend, Avery Gaines, died instantly after being shot in the head as he was sleeping on the floor, Lawhorn wrote, according to the Clarion-Ledger. Lawhorn then walked to his mother's bedroom and tried to shoot her, but his gun jammed, he wrote.
He left to fix the weapon, walked back to the bedroom and fired two shots in the dark, according to another description of the post by ABC News.
"She started screaming the worst scream I've ever heard," Lawhorn wrote. "Movies really don't do justice to how true terror sounds."
He kept shooting at his mother until "it was over," he wrote, adding that he spared his family's dog and cat.
Movies really don't do justice to how true terror sounds
"Thoughts and prayers" are a waste of time, he told his Facebook friends, and would not help his brother, who Lawhorn said he had hurt the most with his actions.
"If you want to help him in this insanely difficult time I'm putting him through, actually do something, don't just think about it. I'm sorry to him more than anyone," he wrote. "That said, I don't want or need forgiveness from anyone. What I did is unforgivable."
"Nothing happens after death, but if there is a hell, I'm going to be in the lake of ice at the bottom."
Lawhorn did not have a violent criminal history. Allen, the East Ridge police official, said authorities are not aware of any previous incidents involving him and his mother.
His vehicle was found on Interstate 59 in Jasper County in Mississippi. He was not in the car, but authorities suspected he was still in the area, the Jasper County Sheriff's Department said at the time.
Where to get help:
• Lifeline: 0800 543 354 (available 24/7)
• Suicide Crisis Helpline: 0508 828 865 (0508 TAUTOKO) (available 24/7)
• Youthline: 0800 376 633
• Kidsline: 0800 543 754 (available 24/7)
• Whatsup: 0800 942 8787 (1pm to 11pm)
• Depression helpline: 0800 111 757 (available 24/7)
• Rainbow Youth: (09) 376 4155
• Samaritans 0800 726 666
• If it is an emergency and you feel like you or someone else is at risk, call 111.
