Philadelphia Detective John McNamee knows of cases that stay with homicide investigators. Cases that remain stubbornly unsolved, cases that sit on the desks of veteran detectives who have cleared dozens of killings. Cases that they carry for years.

He doesn't want to carry Amber Long's case for much longer.

Long was 26 when she was killed walking to her car in Northern Liberties on the night of Jan. 19, 2014.

She and her mother had just attended a gala at the Philadelphia Museum of Art. On a desolate stretch of Front Street, two men approached them and grabbed at their purses.

Long held onto hers a second too long. Her assailant pulled out a .22-caliber pistol and fired once, striking her in her abdomen. Long collapsed on the sidewalk at her mother's feet.

The men fled in a rented car.

McNamee has worked on the case for two years - two years spent poring over surveillance footage, sifting through hundreds of rental-car records, following up on leads that never panned out.

What he needs now, he says, is for someone to "grow a conscience" and call in the tip that will break Amber Long's case. The city's $20,000 reward in the case still stands.

"We have everything we need. We've collected the evidence. We're ready for when the day comes," McNamee said in a recent interview at Police Headquarters.

Long's case is not without evidence. It's just that the evidence hasn't been enough.

There is surveillance footage of the shooting that only shows the killers from behind. Video of the rental car they drove that's too blurry to make out the license plate. Ballistics evidence that has not matched any known gun.

"There were at least two, maybe three, people involved," said McNamee's supervisor, Lt. Mark Deegan. "We're hoping that maybe someone talks too much one night."

They're hoping that someone else overhears an incriminating conversation and decides to call police.

In the meantime, they wait. They check guns seized by local and federal authorities to see if one can be traced to Long's killing. They took the video of Long's killing to the FBI in an effort to enhance the picture. They follow up every tip that comes in, however insubstantial.

McNamee has stopped watching the grainy surveillance video of the shooting.

"I don't have to see that video anymore," he said. "It is ingrained in my mind."

Long was a transplant from Harrisburg, a graduate of Philadelphia University who stayed after graduation and decided to make the city her own. An architect, she had landed a promising job at a local firm just before her death.

In the two years since the shooting, Long's mother, Stephanie, has cleaned out her daughter's apartment on Ritner Street in South Philadelphia. She has carted off Amber's unfinished paintings, and finished some herself. Shehas raised tens of thousands of dollars for a scholarship in Amber's name.

On the first anniversary of her daughter's death, Stephanie Long showed up at the homicide unit with daffodils for the detectives - bright flowers in honor of her daughter's "bright, sunny smile." Daffodils will dot Philadelphia University's campus come spring - Stephanie Long and her daughter's friends planted 1,000 bulbs there in October.

"Hopefully they will bring as much joy to others as they would have to her," she wrote on Facebook at the time.

She could not be reached for comment on Monday. The Facebook page she runs in honor of Amber has been quiet of late, but was flooded with birthday wishes on what would have been her daughter's 28th birthday on Dec. 23.

"So many things make me think of you. Your wisdom and maturity that was beyond your years and your love of life in general. I remember cartwheels in the grass, building snowmen and counting stars," one friend wrote. "Thinking of you today and every day."

Deegan and McNamee, in a cramped back office at Police Headquarters, say they will chase every tip they get, follow every scrap of a lead, be there when, at last, the call they need comes in.

"You don't get frustrated," McNamee said. "You don't lose your determination."

He doesn't want to carry this case. But he will carry it for as long as he must.

Tipsters are asked to call the department's anonymous tip line at 215-686-TIPS or the Homicide unit at 215-686-3334.

215-854-2961@aubreyjwhelan