Two bomb vests were found unexploded in the heavily fortified Green Zone in Baghdad, a military spokesman said Sunday, less than a week after a rocket attack killed two Americans in the vast central area.

U.S. military spokesman Rear Adm. Mark Fox said the vests were found Saturday and the matter was under investigation.

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"It reflects the nature of the security challenge that we're facing," he told reporters, without giving more details.

Two Americans — a contractor and a soldier — were killed in a rocket attack on the Green Zone on Tuesday.

Insurgents and militia fighters routinely fire rockets and mortars into the Green Zone, the nominally secure area in central Baghdad that is site of the U.S. Embassy and the Iraqi government and parliament. The attacks seldom cause casualties or damage because they are poorly aimed and the zone contains much open space.

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