Where were you eleven years ago today? Most of us know exactly what we were doing when we heard the news that a plane had flown into the side of one of the Twin Towers.

Tuesday marks the 11th anniversary of the 9/11 terrorist attacks.

U-S President Barack Obama and first lady Michelle Obama plan to attend the Pentagon ceremony. Vice-President Joe Biden is expected to speak in Shanksville, Pennsylvania.

Security is on high alert in New York City. While there are no specific threats,  federal authorities say they are concerned about potential extremist activity. The security ramp ups have become routine on this date. From Grand Central Station to the World Trade Center site, New York City stiffens its security spine each anniversary of the attacks.

Meantime, the U-S government says it will include about 50 types of cancer on the list of Sept. 11 World Trade Center-related illnesses covered by a program to pay for health coverage. The National Institute for Occupational Safety and Health announced the change on Monday. .

Scientists say there's little research to prove exposure to toxic dust from the destroyed twin towers caused even one kind of cancer, but an advisory panel said recently it was plausible first responders and others who were exposed to the toxic dust might get cancer.