(CBS News) KABUL -- They were just on their way to a wedding when the bomb ripped through their packed minibus. The explosion killed 18 Afghan villagers. Fourteen victims were women. One was a small child.
Nobody needed reminding that roadside bombs are the number one killer in Afghanistan. But dealing with that threat head-on is the reason why U.S. soldiers alone make the final sweep of any road used by American forces. It's one military operation that's not done alongside the Afghan Army.
As tens of thousands of U.S. troops and hundreds of tons of equipment move out of Afghanistan in the next year, U.S. combat engineers will be safeguarding every inch of every road they travel. CBS News joined soldiers of the Army's 8th Engineer Battalion from Fort Hood, Texas, on a route clearance through a volatile province in eastern Afghanistan.
The last time Sergeant Keenan Roberts and his platoon came down this road, they walked straight into an ambush.
"We had been dismounted from the vehicles for a couple hundred meters when we took small arms fire," he said.
Flying bullets strafed both sides of the column of men who were on foot patrol. Roberts said it was a miracle no one got hit. Their training kicked in immediately. They got low and returned fire. A soldier in a nearby vehicle provided cover, blasting the wooded area where the shots came from with rapid bursts of fire from the mounted 50-caliber machine gun.
Roberts said the soldiers never got a good look at the gunmen, who disappeared through the thick reeds and overgrowth in a riverbed.
If you thought all U.S. troops were either packing up and heading home or hunkering down behind the blast barriers and razor wire of their bases, think again. Soldiers go outside the wire every day, and U.S. forces will keep conducting these dangerous operations until the last American truck leaves. You could call them America's exit strategy. They're certainly a big part of it.
Their mission is route clearance, finding and disabling the improvised explosive devices, or roadside bombs. It also means combat engineers from route clearance units will be the last ones to leave.
"We're out here looking to flush out the trigger men," Roberts said in a drawl with more than a hint of his hometown of LaFollette, Tenn. "You're looking for 'ant trails,' any disturbed earth, discoloration, for a command wire. Mostly they use lamp cord wire. We call it angel hair."
While this job has to be done on foot -- every moment keeping an eye out for anybody planning another ambush -- specially designed armored vehicles are combing the dirt roads in search for bombs.
They're called MRAPs, or Mine-Resistant Ambush Protected vehicles. Leading the way is an MRAP with mine rollers in front, like rows of steam rollers, designed to trigger bombs before the vehicles themselves roll over them.
The Buffalo is a bulked-up version with a 30-foot mechanical arm and a camera mounted on its claw. That enables it to peek into culverts, the steel or cement structures that allow water to pass under roads, an easy place for the Taliban to hide bombs without having to bury them in the road.
Another vehicle looks like a big bulldozer, but instead of the scoop in front it has a big metal plate, equipped with ground penetrating radar.
Route clearance units are discovering a lot of bombs. One recent patrol discovered 14 hidden bombs on stretch of road less than one mile long.
Commanding officer Captain Andrew Elliott said he worries "like a parent" every time he sends his soldiers outside the base.
"No matter the training, there's still that threat," he said. "No matter how much body armor you're wearing, that bullet, that fragmentation (from a bomb or grenade) can still find its way to those hotspots that aren't covered. So you just worry, period, about your guys and gals that are on the front line, fighting every day."
It's a fight that's becoming more difficult by the day. As U.S. troops and their coalition allies hand over more of the battlefield to Afghan security forces, they're becoming more concentrated in the areas where they still operate.
That's especially true in the hostile mountainous eastern provinces along the border with Pakistan, where there are a limited number of roads American forces can use to move troops and supplies. The Taliban is aware of this.
"We know we're under a constant watch," Elliott said. "They plant hoax IEDs, waiting to for us to misstep that. If we had done that (our search) incorrectly, there would have been a 200-pound bomb there the next day."
Elliot said another tactic Taliban fighters use is what the military calls "reseeding," or planting bombs behind convoys moments after they pass by, knowing that route clearance crews might be returning to base along those same roads.
"Once we don't have eyes on that area, the TTPs (tactics, techniques and procedures) are for insurgent forces to go right back in and put an IED right in the ground or in a culvert, because they know we've checked it thoroughly," he said. "And unless it's eyes on, it's not deemed safe."
Watch: U.S. troops keep pressure on Taliban in final raids, below.
"Route clearance is always in the lead," Elliot said. "But just last month, there was a unit behind us that stopped for a mechanical issue, and within that time frame they actually put an IED right behind us. Within minutes."
But route clearance engineers can't clear every road, and Afghan civilians make up the overwhelming majority of deaths from roadside bombs, like the attack on Sunday that killed 18 people.
The Afghan government has chosen not to release the figures of Afghan security forces who have died in Taliban attacks this year, but as coalition forces pull back from combat missions in some of the most volatile areas, Afghan soldiers and police have suffered heavy and increasing losses.
CBS News was told they're also finding the majority of the bombs.
But for the U.S. military, protecting its own forces is a priority as this long war winds down and comes to a close, now that almost everyone's so close to going home.
"You want everybody to walk off that plane in Texas with all their fingers and toes," Roberts said on patrol. "We want to go back as a group. We don't want anybody getting hurt and going back by themselves."
Part of that means that U.S. route clearance crews make the final sweep of any road used by American forces, regardless if Afghan security teams have checked them first.
"Route clearance will always be in Afghanistan for the long term," Elliott said. "There will always be route clearance and combat engineers leading the way."