After terrorists hijacked planes on Sept. 11, 2001, and used them to kill nearly 3,000 people in New York, Pennsylvania and the Pentagon, the nation had a hard fact to face: An extreme Muslim group called al Qaeda was willing and able to hurt Americans here at home. Not since the attack on Pearl Harbor in 1941 had the nation felt so threatened at home. Americans responded with a determined effort to protect the country.

The Department of Homeland Security was created and soon began issuing color-coded warnings about the possibility of terrorist activity. Life changed in ways big and small. Locally, schools canceled field trips. Long lines formed at airports while travelers waited to be scanned. Some streets and landmarks such as the Washington Monument were barricaded or closed.

There have been no attacks on U.S. soil since Sept. 11. President Bush says he works hard to keep the country safe.

But terrorism remains a big concern. One of the top questions facing voters in Tuesday's presidential election is: Which candidate -- President George W. Bush or Senator John F. Kerry -- can best protect the country? The two have different ideas about that.

Bush sent troops to Afghanistan to hunt for Osama bin Laden, the head of al Qaeda. Although he has not been captured, three-fourths of al Qaeda's leadership on Sept. 11 has been killed or captured, Bush says, and the group's training has been disrupted and some of its funding cut off.

Bush also sent troops to topple Iraqi president Saddam Hussein, who was not involved in the Sept. 11 plot but was, Bush says, a threat to U.S. security. The United States had to do things differently after Sept. 11, he says, including being willing to take on countries that pose a danger, even if that means attacking them first.

Kerry says Bush should have kept pressing to find bin Laden rather than diverting troops and weapons to Iraq. Americans are now less safe, Kerry says, because the U.S. invasion of Iraq angered people there and elsewhere and has led thousands of people to join anti-American groups.

Kerry says the United States should protect itself, but will be safer if it has the support of other nations in fighting terrorism. He also says Bush has moved too slowly on some changes, such as creating a single list of terrorists to watch for. (There still are several lists, with conflicting and outdated information.)

What it comes down to is figuring out how to stop terrorists before they strike again.

That's a tough task, says a panel created to study Sept. 11; U.S. intelligence agents had many clues about the Sept. 11 plot beforehand, but never fit them together.

They were like a family with a 500-piece jigsaw puzzle, only some people were working on one part in the kitchen, while others had their pieces in the living room. Nobody got the entire picture.

To fix this, the panel recommended naming a national intelligence director, which Bush and Kerry support. This person then might speed efforts to put together a single terrorist watch list or shorten those long security lines at airports and such.

Another challenge is figuring out how to protect America without denying people their rights -- to privacy, for example, or to a fair and speedy trial.

The USA Patriot Act, an anti-terrorism law passed after Sept. 11, gives officials more power to collect information about suspects and make arrests. Supporters say the law is an important tool in fighting terrorism; critics say it has opened the door for government to snoop into people's lives and trample their rights.

-- Fern Shen

Since the attacks

of Sept. 11, 2001, even Uncle Sam has to go through security checks.

In this case, actor Micah Bump was making an appearance at the vice president's residence in 2002.