NEW YORK - President Barack Obama opened a day of solemn remembrance today by honoring those killed on Sept. 11 with a visit to Ground Zero, touching the smooth, bronze panels bearing the names of those lost at the World Trade Center.

Relatives of victims of the Sept. 11 attacks and the 1993 World Trade Center bombing were allowed in to the lower Manhattan memorial for the first time, many carrying photographs and some with flags as they entered the memorial plaza.

In the nation's capital, Defense Secretary Leon Panetta, after a moment of silence at the exact time the Pentagon was hit by a hijacked jetliner a decade ago, paid tribute to those who have served in uniform in the decade since.

He said more than 6,200 members of the U.S. military have died in the wars in Iraq and Afghanistan since the Sept. 11 attacks.

And in Pennsylvania, Gov. Tom Corbett told mourners that the sacrifice made by the 40 passengers and crew aboard United Flight 93, which crashed in a field near Shanksville, helped chart a new course and standard for American bravery.