Sunday marks the fifteenth anniversary of the September 11th attacks. US President Barack Obama has delivered a speech to commemorate this event. Obama honored the memories of the victims of the attacks and urged Americans to remember the "core values" that define them.

"The terrorist threat has evolved, as we've seen so tragically from Boston to Chattanooga, from San Bernardino to Orlando. So in Afghanistan, Iraq, Syria and beyond, we'll stay relentless against terrorists like Al Qaeda and ISIL. We will destroy them and we'll keep doing everything in our power to protect our homeland," said Obama.

Obama was speaking two months before the presidential election in which real estate magnate Trump will face Democrat Hillary Clinton.

Obama said the "terrorist threat has evolved" and urged American to examine how they responded to the threat. The president urged Americans to remain united in the face of terrorist attacks.

The Al-Qaeda hijackings on September 11, 2001 were the first foreign attack on the US mainland in nearly two centuries.  Almost 3-thousand people died in the attacks when the hijackers slammed airliners into New York's Twin Towers, the Pentagon and a Pennsylvania field.