A soldier killed over the weekend south of Baghdad was the first American casualty in a roadside bomb attack on a newly introduced, heavily armored vehicle, military officials said Tuesday.

The death, however, has not changed the Pentagon's mind about its plans to spend more than $22 billion to buy thousands of the mine-resistant, ambush-protected vehicles, known by the acronym MRAP, for the Army and Marine Corps to use in Iraq and Afghanistan, said Pentagon Press Secretary Geoff Morrell.

"That attack has not ... caused anyone to question the vehicle's lifesaving capacity," Morrell said. "To the contrary, the attack reaffirms their survivability."

The soldier who died Saturday was the gunner who sits atop the MRAP vehicle. Morrell said it is still not clear whether he died as a result of the explosion or the rollover. And Maj. Alayne P. Conway, deputy spokeswoman for the 3rd Infantry Division, said the attack and the death are under investigation.

Morrell said the MRAP hit a "very large, deep-buried IED" and that the "force of the explosion blew the MRAP into the air and caused it to overturn." Despite the size of the explosion, he said, the crew compartment "was not compromised" and the three soldiers inside escaped with cuts and broken bones in their feet.

"I think everybody is still amazed at the fact that, despite the size of this bomb, these vehicles are proving to be every bit as strong and as lifesaving as we hoped they would be," said Morrell, adding that Defense Secretary Robert Gates is "more convinced than ever that these vehicles do indeed save lives."

The V-shaped hull of the huge MRAP truck is designed to deflect blasts from roadside bombs, a weapon that has killed more American soldiers than any other tactic used by Sunni insurgents and militia fighters in Iraq.

Although this is the first fatality since the new MRAP program was declared a Pentagon priority and launched about a year ago, there have been three recorded American fatalities in similar vehicles prior to that. In one case, a service member who was not wearing his seat belt was killed when the bomb exploded near the vehicle.

There now are more than 2,225 of the costly vehicles in service in Iraq and Afghanistan, and the Pentagon is working to buy as many as 12,000 more. MRAPs cost between $500,000 and $1 million, depending on their size and how they are equipped. They can weigh between 19 and 40 tons.

Morrell said that commanders in Afghanistan are interested in getting some of the lighter MRAPs, which they said would work well in that terrain.

The sophisticated vehicles are being built and put into service in a bid to provide soldiers and Marines more protection than is offered by armored Humvees, which have flat bottoms that absorb the shock waves from a blast. The bottom of an MRAP also is 36 inches above the ground, while Humvees sit much closer to the roadway.

The MRAPs are not as agile as the Humvees, however, and thus are more cumbersome and not ideal in some battlefield locations.