HAMBURG, Germany, Aug. 10 — A German court today began a retrial of Mounir el-Motassadeq, the only person convicted of involvement in the Sept. 11 terrorist attacks, with the disclosure that the United States will for the first time share evidence about the plot. Mr. Motassadeq's conviction was thrown out in March by an appeals court, which said that critical evidence had been withheld by American authorities. After having been sentenced to 15 years in prison, Mr. Motassadeq was freed in April. The decision by the United States to offer limited cooperation to the Germans introduces a combustible element to...