They were leaving the area when Riley took a step and felt the earth give ever so slightly beneath his right boot.
"The one thing you know in Afghanistan is that, if you're not on solid ground, you're not in a good place," he says. "The minute that ground gave out a little bit, I just swore in my head and I knew exactly what had happened."
Buried beneath a "pressure plate" was a fuel can filled with ammonium nitrate — the same explosive mix used in the Oklahoma City bombing. Riley blacked out "for a split second, but woke up flying in the air and landing."
The blast took off both of Riley's legs, just above the knee, and three fingers on his left hand. He had about a week left on his deployment.
"I was, excuse the pun, I was one foot out the door," he says with a laugh. "It was probably on one of the last patrols I would have done in my deployment."
After more than a year and a half of recovery and rehab, Riley was medically retired from the Marine Corps this summer.
Learning to walk on his prosthetic legs was "like kicking a soccer ball in a swimming pool." But he didn't just learn to walk; he has learned to soar. He joined "Operation Rebound," and has graduated from competitor to mentor.
He's 27 now, living in San Diego, and though he supports the war, he understands the frustration of many who want it to end.
"I've seen both sides of it," he says. "I've seen good being done. I've seen kids going to school, roads being built, bridges being built — that kind of thing. I've also seen a bad side of it. You see seemingly an endless war where you're continually fighting, and it's hard to see progress. ...
"I mean, one casualty or 2,000 casualties," he says. "You know, it's numbers."
The 2,000th casualty occurred at a lonely Afghan Army checkpoint along the main road between southern Kandahar and the national capital of Kabul — an area of scrubby, rolling foothills dotted with Pashtun villages and trees bearing fist-sized, yellow apples.
According to Afghan officials, Metcalfe and his squad were on foot patrol when the checkpoint came under insurgent attack. Believing they were being fired on by their Afghan allies, Metcalfe and the others engaged the checkpoint, the officials said.
Metcalfe, a civilian contractor and at least two Afghan soldiers died in the firefight. The Pentagon is investigating.
Metcalfe, a member of the 173rd Airborne Brigade Combat Team, was an 11-year veteran and was on his third deployment. He leaves a wife and four children, aged 11 months to 12 years.
Lisa Freeman can't help thinking he should never have been in that "impossible place."
"I don't really understand why we're there anymore," she says. "Why are we still watching death after death, and pain after pain here at home?"
All she knows for certain is that her son would not want the United States to leave just yet.
Two days before his death, Freeman called his mother back in Georgia. He told her all about the friendly locals, and how cute the children were.
"The kids would rather have pens and paper more than anything," he said. "Even food or water."
He asked if she would start collecting school supplies that he and the other troops could distribute in the villages.
"Send as much as you can," he told her. "I want to give them this."
She was discussing the first fundraiser with her eighth-grade class at Richmond Hill Middle School when the Marines arrived to inform her of her son's death.
His last request has since grown into the Matthew Freeman Project, "Pens & Paper for Peace." In the past two years, the nonprofit charity has shipped more than six tons of school supplies to military personnel for distribution in Iraq and Afghanistan.
Lisa Freeman says one of the project's volunteers told her recently that it might take years, but that their efforts would bear fruit.