Remembering 9/11

New Yorkers head back to work Monday, after Sunday’s extensive security marked the tenth anniversary of September 11.

Security efforts were also increased in Washington, following last week’s news of a “credible threat” in the two cities.

Thousands of people gathered near Ground Zero for the anniversary.

Only family members of those who died in the attacks were allowed to attend the ceremony, and to go inside the National September 11 Memorial that opens to the public Monday.

In Arlington, Virginia, a huge flag was draped over the side of the Pentagon hit by American Airlines Flight 77.

Friends, family and government dignitaries gathered for a sombre ceremony to remember the 189 people who died on the plane and in the Pentagon that day.

In Shanksville, Pennsylvania, a newly-dedicated memorial was unveiled to honour the 40 passengers who fought back against hijackers.

U.S. President Barack Obama and the First Lady visited the impact site, after attending ceremonies in New York.

Many Canadians also commemorated the anniversary.

A large ceremony took place in Gander, Newfoundland. The town played a huge role after the September 11 attacks.

Residents took in more than 6,000 people when planes had to be diverted from the closed U.S. air space.

Video: Brittany Gogo on the opening of the 9/11 memorial at Ground Zero in New York City: