CAIRO, Egypt -- A claim of responsibility for the downing of two Russian planes appeared on a Web site known for militant Muslim comment Friday. The statement, which accused Russians of killing Muslims in Chechnya, was signed "the Islambouli Brigades." A group with a similar name has claimed at least one previous attack, but the legitimacy of the group and the authenticity of such statements could not be verified. Russian officials have said terrorism was the most likely cause of Tuesday's plane crashes, which killed 89 people. "We in the Islambouli Brigades announce that our holy warriors managed to hijack...