In addition to murder, Jonathan Lewis Herndon, 48, was booked for tampering with physical evidence and abuse of a corpse.
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Police officers were called to check on the welfare of Constance Thomas, but she was found dead in the apartment.
"I mean it's just evil. It's just horrifying. It's just like the devil," Dona Kerger, the victim's best friend, said.
She described what was behind Thomas's door as a nightmare.
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"I said 'Connie is up there dead and I know it and you need to call them or I'm busting this door down,'" Kerger said.
On Friday, Kerger could not get a hold of Thomas, who lived in an apartment upstairs from her own.
That night Herndon told Kerger he didn't have a clue where his girlfriend was.
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"He was like, 'I don't know, but if you see her tell her I'm gone to a friend's house,'" she recalled.
On Sunday, police checked the apartment and found Herndon unconscious on the couch, trying to overdose.
LMPD's Homicide Commander, Lt. Todd Kessinger said he'd written a note, though he declined to describe it.
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At first, he said, Thomas wasn't immediately found.
"You don't expect it to be concealed the way the body was concealed," Kessinger said.
Thomas was later found decomposing - hidden under a pile of clothes in her room. A bag had been placed around her head.
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"It's just devastating because it could have been prevented," Anne Owens said.
Owens is Herndon's former stepmother. She said she knew he'd abused Thomas years before when they first dated. She said Herndon was no stranger to violence against other women.
"It's a classic example of an individual who has been given entirely too many chances," Owens said.
Kerger knew about their violent past as a couple.
"She just kept saying 'he's sick. He's sick. He'll get better,'" Kerger said.
That day didn't come soon enough.
"She didn't deserve it," Kerger said.
Police said Herndon is in custody, but is refusing to talk.
The official cause of Thomas' death was blunt force trauma. The weapon has not been found.
Inside the apartment, authorities also found Thomas' dead dog, Lex. Detectives aren't sure if the dog died because it wasn't taken care of after Thomas died, or if it was purposely killed.
WAVE reports that Herndon is being held on $100,000 bond at Louisville Metro Corrections. He is scheduled to be arraigned July 23.