On Tuesday, Mary McKenna will put a plant with purple flowers and a balloon into her car in Prior Lake and drive downtown to the homicide unit of the Minneapolis Police Department, where she will deliver them to the current detective holding the file of the murder that has haunted her for 20 years.

That's how long McKenna has made the trip, which is both a tribute to her foster daughter, Deltrece Benson, and a reminder to detectives that the person who killed her is still out there.

A landlord found Deltrece on a Saturday night in 1990 in a northeast Minneapolis duplex. She was nude and had two scarves tied around her neck. An autopsy showed she had been strangled. Deltrece, the seventh murder victim of the year, was 12 years old.

Police believed that the killer was someone she knew, and that she let him into the apartment. The girl's mother, Jodi Benson, was out that night but called to check on her. When Deltrece didn't answer the phone, her mother asked the landlord to knock on the door. He found her shortly before midnight.

A few days after the murder, police arrested a 15-year-old boy. They didn't have enough evidence to charge the boy, and released him. Police later asked for the public's help. They know that the person who killed Deltrece took a cable television converter box and a videocassette recorder from her home, but there was little other evidence.

According to articles at the time, Deltrece was removed from her home by child protection workers and placed with McKenna's family in Prior Lake. McKenna said that Deltrece's mother had struggled with various problems but eventually won back custody. While living with her mother, Deltrece continued to spend every other weekend with the McKennas.

"She really thrived," McKenna said. "She was in the Girl Scouts, she played softball and basketball. She had lots of friends and had sleepovers."

Sgt. Chris Hauglid worked Deltrece's case for years. He kept a photo of her on his desk until the day he retired last year, according to Capt. Amelia Huffman. The investigation, and Deltrece's photo, has been passed on to Sgt. Darcy Klund and is considered an active case, Huffman said.

McKenna and others familiar with the case have been frustrated over the years at the lack of progress. "I don't know how you can spend 20 years and have nothing," she said.

Deltrece was buried at Hillside Cemetery, not far from where she lived. Her headstone reads: "Don't worry, be happy," a lyric from a popular song at the time. "It was our motto around here," McKenna said.

"I'm still grieving, and I'll always be grieving," she said.

The murder shook her family just as it did the Benson family. McKenna said her marriage broke up not long after, and her two children, then ages 8 and 9, were traumatized by her death.

McKenna's oldest son, Joey, has her initials tattooed on his chest. He was in a near-miss accident once, and said he felt like Deltrece was watching over him.

"My [second] husband has asked me what would change if they solved it," said McKenna. "I'm 55 years old and I don't want to die with this case unsolved. She deserves better."

So on Tuesday, she will add another symbol of life to the homicide office, and perhaps to a case that means so much to her.

jtevlin@startribune.com • 612-673-1702