On the 1200 block of North Orianna Street, a narrow, almost alleylike road that juts off from Girard Avenue on the edge of Northern Liberties, the walls facing a grassy plot of land have been brightly painted with colorful flowers, butterflies, and stars. The grass is mostly clear of litter, the plantings around the trees are well-maintained, and the air smells sweet and floral.

The spot is oddly quiet. Girard Avenue is just a few steps away, but the sound of traffic is faint, even in daylight. From a spot high on one wall, a black-and-white portrait of the garden's namesake, Sabina Rose O'Donnell, stares with large, dark eyes.

O'Donnell was killed there almost two years ago, a few weeks before her 21st birthday, when the plot was a gravelly, trash-strewn vacant lot. Friends and family have since cleaned out the space, planted daffodils, and painted over graffiti. Last June, on the anniversary of O'Donnell's death, they held a candlelight memorial there.

The trial of O'Donnell's alleged killer, Donte Johnson, is set to start this week in Common Pleas Court. After a hearing Monday that will determine whether Johnson's police confession can be used as evidence, jury selection is expected to begin. The trial could last two weeks, said Assistant District Attorney Richard Sax. Johnson, now 20, faces life in prison.

Johnson and O'Donnell crossed paths by chance, and on a night that seemed at first to offer nothing out of the ordinary for either of them. On June 2, 2010, after going to a couple of Northern Liberties bars and then to a friend's house to watch movies, O'Donnell hopped on a bike and pedaled off in the direction of home, as she had done countless times before.

Johnson, an 18-year-old who lived several blocks away from the apartment O'Donnell shared with her stepfather at Fourth Street and Girard, spent much of the night circling the neighborhood on his bike. He glanced into the parked cars he passed, looking for things to steal. He had no significant criminal record. He had never seen O'Donnell before.

There was nothing, in other words, that could have predicted what authorities say happened next.

After Johnson spotted O'Donnell in the early hours, authorities said, he followed her, yanked her off her bike, and dragged her to the back of her apartment building. He is accused of beating and raping her, then strangling her. A neighbor walking a dog found O'Donnell's nude body the next morning, badly beaten, her bra knotted around her neck so tightly that the underwires dug into her skin.

Johnson was arrested two weeks later, after a tipster led police to him. In a statement he made to police, Johnson said that he wanted to steal O'Donnell's bike but that she screamed, the situation escalated, and he started choking her.

"I don't like the whole thing," Johnson told police. "I shouldn't have did it. I shouldn't have put my hands on her. All over a bike."

Even in a city that sees more than 300 slayings a year, O'Donnell's killing was unusual, given the randomness of the circumstances and the brutality of the attack. Johnson, a high school dropout, had been arrested for drug possession and for punching a school police officer, but police in the area thought of him as a petty criminal.

Mark Rounds, O'Donnell's stepfather, suspects those who loved O'Donnell will always be haunted trying to understand why she was killed.

"I can't reconcile a supposed motive with what happened," he said. "I can't reconcile any method of reasoning with the violence that took place. I'll never understand it."

O'Donnell's death shocked Northern Liberties, sending a ripple effect through a rapidly gentrifying community that many new residents had thought of as safe. O'Donnell, a vibrant, well-known member of the neighborhood, was an aspiring dancer and model who worked at PYT, a popular burger lounge in the Piazza at Schmidts. She was beloved by those who knew her, from the friends she had made in high school to the tellers in the bank where she cashed her paychecks.

Johnson, whose mother could not be reached for comment, lived with his family in a housing project a few blocks north. After his arrest, neighbors spoke well of Johnson's mother, saying she worked hard to support her children. When Johnson first appeared in court to answer to the charges, she wept and was comforted by loved ones. Johnson, too, sobbed through much of the appearance.

Prosecutors initially said they intended to seek the death penalty, and in December 2010, Johnson was on the verge of pleading guilty in a negotiated agreement that would have put him behind bars for life. He abruptly changed his mind, dropped his court-appointed attorney, and hired Douglas Dolfman, a private lawyer who said Johnson and his mother wanted a second opinion. Last year, after it came out in court that Dolfman had not been paid, court-appointed attorneys Gary Server and Lee Mandell took over the case.

Last week, Sax said that the District Attorney's Office had decided against pursuing the death penalty. Johnson's age was taken into account, Sax said, as well as possible mental-health issues.

Server, one of Johnson's court-appointed attorneys, declined to comment on his planned defense, but in a hearing last month, Server said that Johnson suffers from "brain dysfunction" that affects his ability to make deliberate decisions.

Server also indicated that the proceedings had been difficult for Johnson's family.

"It's a complicated case," he said.

Johnson's police confession is just one component of the evidence prosecutors have assembled against him. A surveillance camera at Fourth and Girard captured Johnson as he biked past O'Donnell about 3 a.m., then immediately made a U-turn and began following her. Prosecutors also have said police found ample DNA evidence from Johnson on O'Donnell's body. There may be additional DNA evidence on the clothes Johnson wore the night of O'Donnell's death.

"The DNA is very strong, conclusive, profound, and powerful," Sax said.

After Johnson left O'Donnell, he is accused of taking her keys and entering her building. He ended up in the apartment of a neighbor, then ran when the neighbor awoke, police said.

Mark Rounds said he and O'Donnell's mother were dreading the trial. Relatives are coming from out of town, and Rounds, who plans to be there each day, expects to see some of O'Donnell's many friends in the courtroom, as well.

"I like to think I'm prepared for it," he said. "But I guess we'll see."

Rounds said he was mostly looking forward to the trial's being over. A sculptor and longtime resident of Northern Liberties, he has recently considered leaving Philadelphia. Like many who have lost a loved one to murder, he has long since accepted that the idea of one day getting a sense of "closure" in O'Donnell's death is a myth.

"I don't expect to ever get to that point," he said. "I don't know what I expect will happen when it's over. There's a part of me that says, when this is done, maybe I don't have to be here anymore. But I really don't know how I'll feel."