Marines in Fallujah, Iraq, say Sunni insurgents are smuggling in weapons and fighters.
They're believed to be using aid convoys and ambulances, disguising the flow of weapons.
On Monday alone, U.S. troops uncovered anti-aircraft guns buried in a load of humanitarian aid. They also saw an ambulance pull up next to two insurgents who'd been shot. People got out of the ambulance and picked up weapons, but left the casualties lying there.
A soldier at a checkpoint says one fighter was found hidden in a sack of grain in the back of a truck.
Marines and military police are using sniffer dogs.
But the military says it doesn't have enough personnel or the right equipment to thoroughly check every vehicle entering Fallujah.