As weeks passed and Shavaughn Wallace's killing went unsolved, her mother would wander into her bedroom, still filled with her clothes and her smell, wondering whether the case would go cold.

"I didn't want her to be one of these victims who I'd have to see months and years from now on those bulletins that say they're looking for leads," Carla Robinson said.

She feared her 18-year-old daughter, a business student at Indiana University of Pennsylvania who loved to sing and was four months pregnant with a baby boy, "was going to become a statistic."

Yesterday, after learning that detectives had made an arrest in the May 22 slaying on the North Side, Ms. Robinson said, "my heart jumped, and I burst into tears."

Lamon Street, 18, of Chartiers City, was jailed last night on charges of homicide, homicide of an unborn child and seven counts of reckless endangerment.

Ms. Wallace was sitting in a parked car outside her boyfriend's house in the 200 block of Alpine Street, talking with about a dozen friends, when, police said, Mr. Street emerged from a wooded lot near Pryor Way and started shooting into the crowd.

The group screamed and scattered at the gunfire. Ms. Wallace ran from the car, but was struck in the back and collapsed.

"She was not a target," police Lt. Daniel Herrmann said. "She was at the wrong place at the wrong time."

Detectives were still investigating the motive, and Lt. Herrmann declined to say what led them to Mr. Street, whom they arrested at the Municipal Courts building Downtown, where he had appeared for a preliminary hearing in another case.

"I'm elated," Ms. Robinson said.

At least one witness said he recognized the gunman as Mr. Street, according to a criminal complaint. The same witness told police he believed the shooting was the result of an ongoing feud among gang members of the Brighton Place Crips and North Charles Street Crips, who are "united against" another group, the Hoodtown Mafia, the complaint says.

The witness said that several weeks before Ms. Wallace was killed, he saw Mr. Street, a member of the Brighton Place Crips, on Alpine Street, and "tension developed" between him and a member of the Hoodtown Mafia. The two "gave each other threatening looks and exchanged threatening words, but nothing happened," the complaint said.

Two weeks later, "a Hoodtown member" took a shot at Mr. Street, and "since then, the Crips from Brighton Place and North Charles have been taking shots at members of the Hoodtown Mafia," the complaint said.

In her Hill District home, where photos of Ms. Wallace still lined a mantle and friends continued to memorialize her, Ms. Robinson said the arrest offers some solace, but hardly closure.

She said her daughter did not know Mr. Street, and she wonders why he would aim in her direction.

In grieving, she also has questioned herself.

"I felt like half a woman," Ms. Robinson said. "How am I a good mother, but I couldn't save her?"

On a silver chain around her neck, Ms. Robinson wears a heart-shaped charm bearing a photo of Ms. Wallace's smiling face and the words, "Always With Me. Rest With God."

"I'm not going to sleep until I've found out exactly what's the reason for this," she said. "It's not over."