When police arrived at Selina Knight’s Southeast apartment just before midnight on March 4, 2011, they found her lifeless body on a leather sofa, bloodied from stab wounds. The smell of bleach permeated the apartment, and a disturbing message was scribbled on a wall.

Written in marker, the message read: “You gave me HIV. You gave me AIDS.”

Authorities arrested Keith ­Littlepage-El, 51, about a week later. Littlepage-El was Knight’s boyfriend of about 20 years, and he lived with her until days before the killing. They charged him with first-degree murder in the death of Knight, 36.

Littlepage-El’s murder trial began Thursday in D.C. Superior Court, with prosecutors and defense attorneys outlining their theories of the case in opening statements.

With no DNA evidence, weapon or witnesses, prosecutors said their case would rely heavily on the testimony of Knight’s friends and family, who were told days before her death that she planned to break up with her boyfriend.

Littlepage-El’s cellphone was also used at Knight’s apartment in the 3000 block of Nelson Place SE the evening she was killed, prosecutors said.

Both sides ascribed substantial significance to the message on the wall. Knight was HIV positive, attorneys for Littlepage-El and the government agreed; she had received the diagnosis more than 10 years ago.

Assistant U.S. Attorneys Robert Feitel and Erik Kenerson say Littlepage-El had known that for years. But he killed her, they argued, because she had ended their relationship and kicked him out of her apartment for being emotionally and physically abusive. The scribbling, prosecutors suggested, was an attempt by Little­page-El to shift the investigation’s focus to someone else.

Littlepage-El took notes as he sat, his legs in shackles, during Thursday’s proceedings. His attorneys, Christopher Kemmitt and Anthony Matthews of the District’s Public Defender Service, argued that their client did not kill Knight. Kemmitt told the jury that Littlepage-El, who was not HIV positive, loved her and stayed with her after her diagnosis.

“He supported her and cared for her,” Kemmitt said.

Instead, Kemmitt suggested, the killer probably was someone who knew Knight and wanted to frame Littlepage-El.

Three of Knight’s friends once attacked Littlepage-El before she was killed, Kemmitt said in opening statements. Littlepage-El reported the incident to police, and one of the friends was arrested.

As a result, Kemmitt said, Little­page-El was labeled a “snitch” and was hated in the neighborhood.

“They don’t have any evidence,” Kemmitt said of the prosecutors. “Just a story to tell.”

Kenerson told the jury that Littlepage-El repeatedly called Knight and her family members, threatening her days before she was killed.

Knight’s apartment was broken into before her death. Only a television was stolen, but an urn containing the ashes of her father had been dumped in the toilet and onto the bathroom floor. Little­page-El has since been charged with the break-in.

Littlepage-El also threatened to put up signs bearing Knight’s picture in the neighborhood telling residents about her illness, Kenerson said.

A day after Knight was killed, a picture was posted in a neighborhood store with Knight’s picture and home address with the caption “HIV Positive.”

Knight’s mother, Anna Delores Mays, was the first witness called to the stand Thursday. Mays told the jury that her daughter had contracted the disease from a previous boyfriend who Knight later learned was a heroin user. Knight’s diagnosis was made in 2002, her mother said.

“She kept getting sick, and we didn’t know what was wrong,” Mays said. “That’s when she was tested.”

Mays said that Knight met Littlepage-El when Knight was a teenager and Littlepage-El was 31, and Mays said she disapproved of the “on again and off again” relationship.

The trial is scheduled to resume Monday before Judge Thomas J. Motley.