In the span of just a few hours on Sept. 11, 2001, the world changed for good. Ask most everyone of a certain age if they can recall where they were and what they were doing on the morning of the Twin Tower attacks in New York City, and they'll likely be able to describe the precise moment they learned of the news.

That included a lot of rock musicians, some of whom, like many Americans across the country, turned on their televisions to watch the unthinkable tragedy unfold live. Others were physically in New York at the time of the incident. Paul McCartney had been sitting in a grounded plane on the tarmac of JFK airport.

“Out of the window on the right hand side of the airplane, you could see the Twin Towers," he later remembered. "First there was a plume of smoke and then there was a second. I said, that’s an optical illusion. It’s probably just some sort of little fire. Finally the steward came over to me and said, ‘Look, something serious has happened in New York and we’ve got to get you out of here.’”

Not long after that, McCartney was one of several musicians to participate in a large-scale benefit show, The Concert for New York City, on Oct. 20 at New York's Madison Square Garden, just a few miles uptown from where cleanup of ground zero was still underway. Organized by McCartney himself, the show included notable appearances from the WhoMick Jagger and Keith Richards of the Rolling StonesDavid Bowie, Elton John and Eric Clapton — Britons who deeply sympathized with their U.S. allies. A month prior on Sept. 21, a televised benefit, America: A Tribute to Heroes, featured performances from Tom Petty, Stevie Wonder, Neil Young, U2, Willie Nelson, Sting, Paul Simon and others. The broadcast was opened by Bruce Springsteen, whose song "My City of Ruins" which was written a year earlier, suddenly took on a more prescient meaning.

In the days, months, and years that followed 9/11, the memories and emotions tied to that singular morning appeared in the work of many musicians. On the 20th anniversary of the Twin Tower attacks, UCR is revisiting various reactions from across the rock community.

How 20 Different Musicians Reacted to 9/11