When Carl Diener was fatally beaten and stabbed on a Clarendon street in 2009, Patti Diener didn't understand how her tall, strong older brother -- a retired government worker who was working at a gym -- couldn't fight off an assailant.

On Monday, she learned part of the reason: Police believe he had two attackers.

Arlington County police said 20-year-old Roger K. Clark III, of Severn, and 24-year-old Javon Martin, of the District, are charged with murder in Carl Diener's slaying on Dec. 29, 2009. Diener, 57, was killed on the 3200 block of N. 13th Street while walking to his job at the Arlington Sport and Health Club.

"It would have taken more than one person," said Patti Diener, who lives in Boise, Idaho. "It never made sense that he was not able to defend himself against an attacker."

Authorities don't believe Diener knew his assailants, said Det. Crystal Nosal, an Arlington County police spokeswoman. She said "evidence took us to them" and there were no apparent connections between Diener and the two men charged in his death.

Nosal declined to say what evidence pointed to Clark and Martin, and would not comment on a motive in the case.

Patti Diener said she had never heard their names before the news of the arrests.

"There was nobody out there gunning for him," she said. "He was just in the wrong place when two young men for whatever reason decided to act on an impulse."

Clark was apprehended last Monday in Montgomery County and Martin was taken into custody in the District on Wednesday, Nosal said.

Diener's slaying stunned his friends, family and colleagues, and a $25,000 reward was offered for information that led to his killer. Nosal said she didn't think anyone was receiving the reward because there wasn't a tip that led to the arrests.

Arlington County Police Chief M. Douglas Scott said in a statement that the charges show "there are no 'cold cases' in Arlington; we continue to investigate crimes long after they occur."

Patti Diener said she is glad the arrests mean more information will come to light about her brother's slaying. But she said she was struck by the suspects' young ages.

"Their lives are forever changed because of their decision that night," she said.

ebabay@washingtonexaminer.com