The Humvee driver, in his final moments, didn't know what hit him. Neither did the U.S. Army.

When a makeshift roadway bomb killed Spc. Joel Bertoldie in Fallujah four years ago, it was the opening salvo in what has grown — from Baghdad's deadly streets to North Carolina's "IED Expo" — into a multibillion-dollar challenge for a U.S. military no more prepared for it than was the young soldier from Missouri.

New statistics show that improvised explosive devices, more than ever, are becoming the Iraqi resistance's weapon of choice, claiming a growing share of American lives. The people dealing with the mayhem attest to it.

"The bad guys are getting smarter, using larger explosions and better explosions," said Capt. Bruce Wheeler, an Army medical officer at the U.S. military hospital at Balad Air Base, north of Baghdad. "Business is up for us. We're seeing a lot of big stuff" — severe injuries — "come through."

In the May-July period this year, the number of U.S. military deaths from IEDs soared to 203, accounting for 66 percent of all U.S. fatalities, according to the authoritative Web site icasualties.org, which tracks military casualties in Iraq.

Those numbers have climbed steadily from the same three-month period in 2004, when 54 Americans were killed by IEDs, 31 percent of total fatalities.

Since Bertoldie's death in July 2003, the first recorded by icasualties.org as IED-caused, at least 1,509 Americans have been killed in Iraq by the makeshift roadside bombs, out of a total 3,707 fatalities.

The daily number of IED attacks has increased six-fold since 2003, the Pentagon says. On one day in May, 101 of the 139 anti-U.S. attacks involved IEDs.

The strategists before the 2003 invasion would have been surprised.

"The ground force in Iraq had not foreseen this threat in the initial planning for Operation Iraqi Freedom," a recent study at the U.S. Joint Forces Staff College found. In fact, the U.S. invasion force's failure to secure Iraq's ammunition dumps in 2003 left tons of bomb ingredients available to insurgents.

The Pentagon has sought to recover via a crash program — the Joint Improvised Explosive Device Defeat Organization, or JIEDDO — that by next year is expected to have spent some $13 billion on detectors and robots to defuse bombs, vehicle armor, training and other means to "defeat" the homemade weapons.

That sum is comparable, in inflation-adjusted dollars, to what the U.S. spent building the two atomic bombs dropped on Japan in 1945, based on figures compiled by Washington's Brookings Institution. Some in Congress complain the money's accomplishing little.

"We don't mind spending money if it's saving soldiers' lives," said Rep. James Moran, D-Va., a member of the House Appropriations Committee. "But we haven't seen that it has saved a lot of lives yet, and it's been up and running for three years," growing from a task force of a dozen to an agency with an authorized staff of 358.

Frustrated lawmakers have turned to micromanaging the effort, stipulating in budget language, for example, that by Nov. 1 all U.S. vehicles on Iraqi roads be protected by radio jammers blocking signals that detonate insurgent bombs. They've also pressed to speed delivery of thousands of "MRAPs," $1-million-apiece troop carriers whose V-shaped undercarriages are designed to deflect the blast of IEDs.

Congressional auditors, meanwhile, are reviewing JIEDDO's operations, and the Pentagon itself this year formed an outside task force to monitor the organization.

At JIEDDO's offices outside Washington, the frustration sometimes lies in getting its accomplishments recognized.

"In regards to defeating the device, we have done a fantastic job," said Army Col. Michael Mahoney, JIEDDO's operations chief. "The enemy is required to put out a lot more IEDs today to achieve a casualty than he had to do previously."

The experts estimate half the IEDs planted in Iraq are now detected and disarmed. To achieve that, JIEDDO has helped deploy an arsenal of tools: add-on kits to "up-armor" Army and Marine Humvee vehicles; small camera-equipped blimps and unmanned aerial vehicles to spot bomb-planting activity; "Warlock" frequency jammers aboard vehicles; bomb-disposal robots; bomb-sniffing dogs.

Out on Iraq's roads, one sees countless "Rhinos" — spindly arms extending far in front of Humvees' bumpers, bearing "hot boxes" whose heat prematurely trips IEDs' infrared detonation sensors. By October, JIEDDO will test another innovation: advanced ground-penetrating radar to detect deeply-buried explosives.

Such technology was on display in June at what was billed as an IED Expo in Fayetteville, N.C., where 75 vendors sought to sell military customers products ranging from surveillance cameras-on-a-pole to MRAP vehicles.

In the end, however, "technologically there is a limit as to what you can achieve," said JIEDDO's Mahoney. "The next piece is, how are you doing at attacking that network, going after the emplacer, the builder, the supplier?"

Those networks are evolving, finding new detonation techniques, new sources and ways to assemble explosives, to join the array of artillery shells lashed together, propane tanks stuffed with explosives, old antitank mines that they've been using. Last month, U.S. troops found an IED fashioned to look like a section of roadside curb.

The deadliest evolution has been toward use of "EFPs," explosively formed penetrators, devices that fire semi-molten slugs of metal that penetrate thick armor. The penetrators accounted for roughly 5 percent of IED attacks in July, but for 30 percent of U.S. combat deaths, said JIEDDO spokeswoman Christine Devries.

Although both Sunni Muslim insurgents and Shiite militias have used IEDs against American forces, the U.S. military says the sophisticated EFPs are more commonly deployed by Shiite groups that, it claims, obtain them from Iran.

The IED evolution sometimes goes in reverse. Because of the success of frequency-jamming from aircraft and ground vehicles, the insurgents' use of radio-controlled detonation has dropped since early 2006 from 60 percent of IEDs to 15 percent, said a knowledgable U.S. officer in the region. The bombers have instead reverted to wire detonation.

"It's sort of gone in a circle," said Mahoney.

It's a circle that demonstrates what may be an open-ended ability of Iraqi IED-makers to keep the world's most powerful military off balance.

In a National Defense University research paper, Mahoney's predecessor at JIEDDO concludes the IED is here to stay, in a looming "Long War" against foes beyond Iraq. As for the Iraqi foe, Col. Bill Adamson writes that he "proved to be quick learning and innovative." And as for JIEDDO, he says its efforts "are not producing the effects desired."