A man has pleaded guilty to killing Carl Diener, a popular community fixture in Arlington County.

Roger K. Clark III, 20, of Severn pleaded guilty to first-degree murder Tuesday in Arlington County Circuit Court, several days before his trial was to start. Diener, 57, was attacked and stabbed in 2009 as he walked from his home to the nearby Sport & Health Club in Clarendon at 2:50 a.m., according to his sister, Patti Diener Lough. It was an unusual killing in Arlington, which recorded no homicides last year.

Clark was arrested in connection with a robbery and assault in Annapolis about three months before Diener was killed, according to online court records. Investigators matched his DNA, found in the pockets of Diener’s sweatpants, to DNA captured by police in Maryland, Lough said she was told by investigators.

Lough said police told her that Clark did not serve prison time in the Annapolis robbery. If he had, she said, her brother would not have been killed.

“I have to relive that every day,” Lough said. “This could have been avoided.”

Still, Lough said she was pleased.

“From the first minutes of this terrible event, we have had an amazing team of police and prosecutors working tirelessly to answer the question of what happened to Carl that morning,” she said.

Another man, Javon Martin of the District, was arrested in the case with Clark, but he was released from custody. Police declined to offer any details about Martin.

“We can’t comment on the case,” said Arlington County police spokeswoman Crystal Nosal.

Clark is scheduled to be sentenced in April. His plea relieved Diener’s family, said Lough, because relatives did not want to experience the emotional difficulty of the trial and be forced to hear details of the killing.

“We hoped it would happen this way,” Lough said. “Nothing — not even this guilty plea — can ever bring Carl back,” she said. “But we are immensely comforted to know that a murderer is in custody and will not be able to hurt another person or family like he shattered ours.”

Diener, who was popular in his Lyon Village neighborhood, was known for his commitment to athletics and health.

“It was a terrible incident, but it led to a lot of positive things,” said H.K. Park, president of the Lyon Village Citizens Association. “It raised people’s awareness and was a good way to spur people to talk more broadly about safety in general.”

Since Diener was killed, the association identified and fixed more than 90 broken streetlights and distributed 1,000 fluorescent bulbs so residents could keep their porches lighted at night.

Lyon Village Park also has a bench and a tennis backboard that are inscribed in Diener’s memory. The tennis backboard was paid for with funds raised by Girl Scout Addison Snidle, who lived two blocks from where Diener was killed.