Fourteen Marines were killed yesterday when their lightly armored amphibious assault vehicle was blown apart by an explosion while they were en route to retake an area in western Iraq that had been cleared of insurgents at least once before.

The attack was the deadliest roadside bombing against U.S. troops since the insurgency began, and came on the heels of an ambush in the same area Monday that killed six Marines, five of them from the same Ohio-based battalion as those killed yesterday.

One Marine survived with unspecified injuries.

Officials in Washington said they didn't know whether the explosion, which also killed an Iraqi civilian translator, was caused by a mine or by an improvised explosive device, or IED, jury-rigged from castoff munitions. IEDs have become the No. 1 cause of death for American troops in Iraq.

In the past two weeks, at least 31 U.S. soldiers and Marines have died in roadside bombings, including explosions that struck Humvees that had additional armor and Bradley Fighting Vehicles, which are more heavily armored than the assault vehicle the Marines were riding in yesterday.

Nine of the dead Marines were members of Lima Company — part of the 3rd Battalion, 25th Marine Regiment, a reserve unit from Ohio that had been sent to the Syrian border to play a lead role in shutting off the main route for foreign gunmen and suicide bombers entering Iraq.

"They used to call it 'Lucky Lima,' " the company's commander, Maj. Steve Lawson, said in May after an urban ambush and a roadside bomb on successive days had killed or wounded every member of one of the Lima Company's three squads. "That turned around and bit us."

As it turned out, Lima Company and the rest of the 25th Regiment were beginning four months of bombings and ambushes in the grimy Euphrates River towns where U.S. commanders say foreign insurgents had moved freely.

As a whole, the 25th Regiment suffered 20 dead in May and June. Then, with yesterday's bombing and two attacks Monday, the number of dead more than doubled to 41.

Gunmen killed at least five members of the regiment on a foot patrol Monday just outside Haditha, the U.S. military said, and the body of a sixth Marine was found a mile or two from the firefight. The same day, a suicide bomber killed a Marine in the town of Hit, about 45 miles from Haditha, the military said.

The Ansar al-Sunna Army asserted responsibility for the killing of the six Marines on Monday and suggested in an Internet statement that it had killed some by knife.

Hit, Haditha and a string of other towns along the Euphrates are populated almost entirely by farmers and merchants of Iraq's Sunni Muslim minority. The vast majority of Iraqi insurgents are Sunnis, and they have been joined by radical Sunnis from other countries who enter Iraq from Syria to attack U.S. and Iraqi security forces and members of Iraq's newly dominant Shiite majority.

With experience, insurgents have figured out increasingly lethal ways to rig bombs, U.S. military officials say. Frequent patrols — which reduce the time insurgents have to plant bombs but put Marines at risk of hitting bombs themselves — have been one of the Marines' main methods for keeping bombers from making entire regions impassable.

Officials in Washington acknowledged that the Marines were riding in a vehicle designed 40 years ago for a different kind of warfare — the beach assaults the Marines made famous at Iwo Jima and elsewhere in the Pacific during World War II.

The AAVP7A1 "Armored Assault Amphibious Fully-Tracked Landing Vehicle" is essentially a modern variant of the WWII model, designed to carry troops from ship to shore and for use in land operations, according to a Marine Corps fact sheet. They weigh 28 tons and have inch-thick aluminum armor. Those in use in Iraq have extra steel armor. It travels at about 6 mph through surf and sand and can cruise at about 20 mph on land.

But those kinds of missions were "combined-arms" assaults, backed by heavy amounts of air, ship and artillery support, said Daniel Goure, a military analyst at The Lexington Institute, a policy group based in Alexandria, Va.

The vehicle wasn't designed to engage in the kind of day-to-day patrolling of insurgent-controlled territory that the Marines are doing in western Iraq. During the initial assault into Iraq, many of the vehicles broke down on the long drive to Baghdad and had to be towed.

"It is very lightly armored. It is under-powered. It is essentially a big boat on land, and that makes it vulnerable," said Goure.

While many lawmakers have been clamoring for the Pentagon to add more armor to vehicles in Iraq to counter bombs, Goure said the issue isn't that simple.

"There are not enough main battle tanks in the world to equip the forces in Iraq," he said. "There is no way to equip these forces to withstand a direct hit from a 1,000- to 2,000-pound IED.

"This is a better choice, frankly, than to put them in Humvees, armored or not, and certainly better than to put them in unarmored trucks," he said. "What it speaks to is the reality of having to fight a war without always having the appropriate equipment in the right numbers in the right places."

"Clearly an AAV does not offer the same protection as a tank does," said U.S. Army Brig. Gen. Carter Ham, who once commanded U.S. forces in Mosul and is now deputy director of operations for the Joint Chiefs of Staff. "But nonetheless, it is an armored vehicle, and the commanders make an assessment as to what equipment is appropriate for each operation."

Compiled from Knight Ridder Newspapers, The Washington Post, Chicago Tribune and Los Angeles Times