Authorities have increased security in New York City and Washington, as U.S. officials investigate a possible terrorist threat timed to coincide with the September 11 anniversary.

In New York, police have set up checkpoints at bridges, tunnels and subway stations. Bomb-sniffing dogs have been deployed across the city, while surveillance cameras are scanning key areas, including Ground Zero, the site where the World Trade Center towers once stood.

In Washington, officers are prohibiting tourists from gathering near the White House, and armed guards are also stationed at the Kennedy Center, where President Barack Obama and first lady Michelle Obama will attend a memorial concert later Sunday.

Al-Qaida terrorists flew planes into the World Trade Center and the Pentagon, just outside Washington, on September 11, 2001.

The high level of security follows an intelligence report detailing a “credible but unconfirmed” threat of an al-Qaida plot to attack the United States during the anniversary. Authorities believe three individuals may have been tasked to carry out attacks using car bombs.