U.S. military officials, analysts and militants themselves say insurgents have learned to adapt to U.S. defensive measures by using bigger, more sophisticated and better-concealed bombs known officially as improvised explosive devices, or IEDs. They are sometimes made with multiple artillery shells and Iranian TNT, sometimes disguised as bricks, boosted with rocket propellant, and detonated by a cell phone or a garage door opener.

In the first six months of battle in Iraq, only 11 soldiers -- about 4 percent of the 289 who died -- were killed by homemade roadside bombs. In the last six months, at least 214 service members have been killed by IEDs, or 63 percent of the 339 combat-related deaths and 53 percent of the 400 U.S. fatalities, according to data complied by the Brookings Institution's Iraq Index.

The development of shaped charges appears to be a direct response by insurgents to the Americans' use of more heavily armored vehicles, according to soldiers and U.S. military explosives experts. Those vehicles -- principally five-ton, armor-plated Humvees -- are used by all U.S troops traveling outside military bases. The Pentagon drew criticism last year for failing to provide adequate protection for soldiers patrolling Iraq's increasingly dangerous streets.