NEW YORK — With bagpipes and somber bells sounding a sharp counterpoint to the commemorative moments of silence, the nation on Tuesday marked the 11th anniversary of the terrorist attack that brought down the World Trade Center in Manhattan and shattered the country’s political psyche.

The sun rose on a cool, crisp morning, remarkably similar to that which dawned 11 years ago. At all three sites — the National September 11 Memorial & Museum in Manhattan, the Pentagon, and a field in Shanksville, Pa. — the focus was on the victims who died when terrorists hijacked four commercial jetliners. The Manhattan ceremony also honored the six people killed on Feb. 26, 1993, when attackers set off a truck bomb beneath the North Tower of the World Trade Center.

A total of 2,983 people died in the 1993 and the 2001 attacks, the latter of which brought down both World Trade Center towers.

Now, the footprint of each tower is filled with a giant reflecting pool, and the names of victims are etched into bronze parapets, allowing visitors to run their hands over the names as they peer into the watery voids.

There were no political speeches in New York, and this year’s presidential campaigns were briefly paused. Both campaigns pulled some negative ads in a bow to the somber ceremonies.

But both candidates were highly visible. President Barack Obama and his wife, Michelle, observed a moment of silence on the White House lawn at 8:46 a.m. Eastern, the moment the first hijacked plane crashed into the World Trade Center. The Obamas then went to the Pentagon, one of the other targets of the al-Qaida attack.

Romney was at Chicago’s O’Hare Airport, where about two dozen firefighters gathered on the tarmac for a brief memorial timed to the moment the first plane struck the World Trade Center.

Standing on and in front of several fire trucks, lights flashing, the firefighters stood at attention, saluting, for a full minute at 7:46 a.m. Central time. A large American flag was draped on the trucks behind them.

Romney arrived at the airport for a campaign flight about 15 minutes later. Before boarding his plane, he strode about 100 yards across the tarmac to where the firefighters were still gathered in a row and shook hands with each of them.

“On this most somber day, those who would attack us should know that we are united, one nation under God, in our determination to stop them and to stand tall for peace and freedom at home and across the world,” Romney said in a written statement distributed by the campaign.

Vice President Joe Biden attended the ceremony in Shanksville, Pa. It took place before a white marble wall with the names of the 40 passengers and crew from United Flight 93, forced to crash in the field by the heroic passengers and crew on the hijacked plane. The names of the dead were read aloud with two bells sounding each time.

“We wish we weren’t here. We wish we didn’t have to be here. We wish we didn’t have to commemorate any of this,” Biden said.