NEW YORK (AP) — Americans are commemorating 9/11 with mournful ceremonies, volunteering, appeals to "never forget" and rising attention to the terror attacks' extended toll on responders.

A crowd of victims' relatives is expected at ground zero Wednesday, while President Donald Trump is scheduled to join an observance at the Pentagon. Vice President Mike Pence is to speak at the third attack site, near Shanksville, Pennsylvania.

Former President George W. Bush, the commander-in-chief at the time of the 2001 attacks, is due at an afternoon wreath-laying at the Pentagon.

Eighteen years after the deadliest terrorist attack on American soil, the nation is still grappling with the aftermath at ground zero, in Congress and beyond. The attacks' aftermath is visible from airport security checkpoints to Afghanistan. A rocket exploded at the U.S. embassy as the anniversary began in Afghanistan, where a post-9/11 invasion has become America's longest war.

"People say, 'Why do you stand here, year after year?'" Chundera Epps, a sister of Sept. 11 victim Christopher Epps, said at last year's ceremony at the World Trade Center. "Because soldiers are still dying for our freedom. First responders are still dying and being ill."

"We can't forget. Life won't let us forget," she added.