Hijacked United Airlines Flight 175 (L) flies toward the World Trade Center twin towers shortly before slamming into the south tower (L) as the north tower burns following an earlier attack by a hijacked airliner in New York City, in this September 11, 2001 file photo. Monday, September 11, 2006, marks the five-year anniversary of the attacks on the United States that killed nearly 3,000 people. (Reuters)