Get ready for a week of somber reminders about a day that changed American life forever. Sunday is the 10th anniversary of Sept. 11, 2001 terrorist attacks, a milestone being observed by schools, houses of worship, towns and cities, state leaders, the president and every form of news media. Classroom lessons will look at the impact. Memorials with steel from two New York office towers will be dedicated. Victims' relatives will share reflections. Politicians and security experts will talk about what has changed and what hasn't.

One thing that hasn't changed is concern about threats from a radical group in the Middle East called al-Qaeda, which hijacked three U.S. passenger jets and killed nearly 3,000 people on that awful September morning. Security is heightened at airports and across the country, especially in New York. The federal government last Friday issued a rare global travel alert, advising Americans that the group still has "the intent and capability to carry out attacks against the United States" and that "significant dates on the calendar" may be targeted.

In the highest-profile observance, President Obama and former President George W. Bush will mark Sunday's anniversary at the Lower Manhattan site called Ground Zero. That's where the impact of two planes caused the World Trade Center to collapse, killing 2,750 people. Another jet hit the Pentagon just outside Washington, D.C., killing 184. Passengers fought back aboard a third hijacked jet headed to Washington, causing it to crash in a Pennsylvania field. Memorial ceremonies also will be held at each of those sites.

State Department warns: "In the past, terrorist organizations have on occasion planned their attacks to coincide with significant dates." -- 'Worldwide Travel Alert,' Sept. 2

Professor says: "We have a new generation for whom this is a story. They know it's an important story, but they just don't know exactly why." -- Randy Roberts, historian at Purdue University in Indiana

Psychologist says: "The anniversary is going to stimulate memories and anxieties. Some people will probably embed themselves in the media and it will be almost like an addiction reliving this." -- Sharon Brennan, mental health counselor, New York City