A judge ordered Medina, who is charged with first-degree murder, be held on no bond. During his court appearance, he wore a vest that prevents him from harming himself.
Medina said little during his appearance besides answering the judge's and public defender's questions. His next court appearance is Aug. 30. He is now being held in jail and on suicide watch.
Medina drove to the South Miami Police Department and told officers he had killed his wife, police said. According to investigators, there were no 911 calls made.
Officers then went to a home and found the body of 26-year-old Jennifer Alfonso inside a townhouse kitchen with multiple gunshot wounds.
Officials say Medina, 31, claimed him and Alfonso were in a fight when he went upstairs to his bedroom and got a gun. Medina said he pointed the gun at Alfonso and she left the room.
Medina claimed that minutes later, Alfonso returned to the bedroom to tell him that she was leaving him. Medina said that's when he followed her without the gun and she began punching him.
According to the arrest affidavit, Medina said he went back to the room to rearm himself with the gun, and returned to the kitchen where Alfonso was armed with a knife. He said he got the knife from her, she began punching him again, and that's when he shot her.
Medina's Facebook posts
Medina posted a picture of a body and an apparent confession to his Facebook page (pictured below) about noon Thursday. Neighbors identified pictures of him as the man who lived in the townhouse.
The photo showed a woman wearing black leotards with her knees bent backwards. Blood covered her left cheek and arm. Medina tagged the photo as Alfonso and captioned it "RIP."
"What sick person goes and writes a status about it and says it's something they have to do?" said Daysi Fernandez, Alfonso's best friend. "He killed her and he posted it."
Miami-Dade police say Medina made both posts, which had hundreds of shares by Thursday afternoon. His Facebook page was removed about five hours after the shooting.
Several community organization pages named Derek Medina appeared on Facebook on Friday afternoon, including one with more than 1,000 likes. Some posted his mug shot while others mentioned his self-published self-help books. The pages asked that friends share them to spread spousal abuse awareness.
Friends: Couple had troubled relationship
Alfonso's friends said the couple had a troubled relationship, but stopped short of saying it was violent. Records show Medina and Alfonso married, divorced, then remarried about three months ago.
Neighbors said officers brought a girl covered in a blanket out of the townhouse. Friends said she was Alfonso's daughter from a previous marriage.
"When they came here, they rushed inside the house and we didn't know nothing was going on and then they carried a little girl out to the car and they were asking her questions," said Oliver Thomas.
KSAT sister station WPLG in Miami was told Alfonso's daughter is staying with her father.
Neighbors added that the man acted like the neighborhood watch captain in the community and carried a concealed weapon. They painted a picture of a paranoid man who kept his windows covered and had surveillance cameras pointed at his front door.
"The guy was kind of sketchy, you know, walking around with a gun," said Yoshi Dade. "It's kind of scary. You never know."
"The windows are not even open. There are cameras all over the place," added Paula Traviño. "He's just into himself. His wife just came in and out. That's all I saw."
Medina's paranoia went beyond his home. The 31-year-old wrote a series of e-books with long titles that dealt with relationships and his experience with aliens.