The Bush administration is reporting (paragraphs are from a New York Times article):

Some of the Army mechanics captured on Sunday after they took a wrong turn in the Iraqi town of Nasiriya were apparently executed by their captors, probably in front of townspeople, American officials charged tonight.

The officials cautioned that the information was based on one source, apparently a communications intercept, and that they were seeking corroborating evidence. It is unclear how many of the seven soldiers were executed, rather than killed in fighting, as the Iraqis contend. Five other Americans were taken prisoner and at least three were still missing.