A soldier killed over the weekend south of Baghdad was the first American casualty in a roadside bomb attack on the newly introduced, heavily armored MRAP - Mine-Resistant, Ambush-Protected vehicle, a military spokeswoman said Tuesday. The hull of the huge armored truck is v-shaped, 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. The soldier who died Saturday was the gunner who sits atop the MRAP vehicle. Three crew members tucked inside the cabin were wounded. The vehicle rolled over after the blast and it was not clear how the gunner died - whether from wounds in the explosion or in the subsequent roll-over. Maj. Alayne P. Conway, deputy spokeswoman for the 3rd Infantry Division, said the attack and the death were under investigation. There now are more than 1,500 of the costly vehicles in service in Iraq and the Pentagon is working to get at least 12,000 more into the theater, using $21 billion provided by Congress.