Iraqi authorities discovered at least 87 corpses — men shot to death execution-style — as Iraq edged closer to open civil warfare. Twenty-nine of the bodies, dressed only in underwear, were dug out of a single grave Tuesday in a Shiite neighborhood of Baghdad.The bloodshed appeared to be retaliation for a bomb and mortar attack in the Sadr City slum that killed at least 58 people and wounded more than 200 two days earlier.