As bells tolled solemnly, Americans marked the 12th anniversary of the Sept. 11 terrorist attacks on Wednesday with the reading of the names, moments of silence and serene music that have become tradition, SRNNews.com reports. At a morning ceremony on the 2-year-old memorial plaza at the site of the World Trade Center, relatives recited the names of the nearly 3,000 people who died when hijacked jets crashed into the twin towers and the Pentagon and near Shanksville, Pa., as well as the 1993 trade center bombing victims' names. In Washington, President Barack Obama, joined by first lady Michelle Obama, Vice President Joe Biden and wife Jill Biden, and members of the White House staff, walked out to the South Lawn at 8:46 a.m. -- the moment the first plane struck the south tower in New York. Another jetliner struck the Pentagon at 9:37 a.m. "It is an honor to be with you here again to remember the tragedy of 12 Septembers ago, to honor the greatness of all who responded and to stand with those who still grieve and to provide them some measure of comfort once more," Obama said. "Together we pause and we give humble thanks as families and as a nation." The 12th anniversary arrives amid changes at the Flight 93 National Memorial in Shanksville, where officials gathered Tuesday to herald the start of construction on a visitor center. At the Pentagon, plans called for a morning ceremony for victims' relatives and survivors of the attacks and an afternoon observance for Pentagon workers. Around the world, thousands of volunteers have pledged to do good deeds, honoring an anniversary that was designated a National Day of Service and Remembrance in 2009.