U.S. flags flew at half-staff in Arizona Wednesday in remembrance of those who died in the terrorist attacks of Sept. 11, 2001.

Gov. Jan Brewer ordered the flags on all state and public buildings to fly at half-staff, and she said she encouraged businesses and individuals to do the same.

It is 12 years since the terrorist attacks on the World Trade Center towers in New York City and the Pentagon in northern Virginia. A third attack was thwarted by passengers on an airliner that crashed in rural Pennsylvania.

Officials in several cities around the state planned remembrances on Wednesday. In Phoenix, officials planned to post a U.S. flag that flew over Ground Zero at the World Trade Center in New York City.

In Washington, President Barack Obama observed a moment of silence on the White House South Lawn at 8:46 a.m. EDT Wednesday, the moment the first plane hit. Obama and others then attended a memorial service at the Pentagon.

In New York, people gathered at the site of the World Trade Center to read the names of the nearly 3,000 people who died there.

In Shanksville, Pa., bells rang and relatives of those killed on United Airlines Flight 93 read their loved ones' names aloud. That flight was bound for San Francisco, but federal officials have said the hijackers intended to crash it into the U.S. Capitol or the White House.