Twenty years ago on Tuesday, Sept. 11, 2001, Merwynn Pagdanganan's day began like every other routine workday at the Pentagon's TRICARE Health Clinic.

But at 9:02 a.m., as he walked around the Pentagon's Medical Command Suite, he heard the news that two planes had struck the World Trade Center in New York City.

At about 9:37 a.m., he noticed the lights started to flicker and he felt "some kind of a mild blast." At first, the blast didn't alarm him because the Pentagon was undergoing renovations and "slight shakes were typical."

"But I noticed that something was different, odd, when the lights flickered in the hallway and in the command suite," he recalled.

"I knew then something was up, something was wrong."

Pagdanganan, a federal health care IT specialist, is one of many in the military medical community who were at the Pentagon when hijacked passenger planes hit the World Trade Center's Twin Towers in New York City and the Pentagon in Virginia; with a final hijacked plane being forced to crash into a field near Shanksville, Pennsylvania.

Most Americans remember the terror, the sense of disbelief, and the confusion that followed. But many also remember the sense of duty, selflessness, and courage that took over individuals and communities across the United States and the world, inspiring them to do something positive to help.

On the 20th anniversary of that day, the Defense Health Agency recognizes, remembers, and honors those who perished in the attacks as well as the survivors and everyone who contributed selflessly to serve others and lend a hand in whatever way possible on that day and in the weeks, months, and years that followed.

For Pagdanganan, the experience profoundly influenced the way he's lived life since then.

"I have so many lessons from that day," he said. "I always tell my family, my kids and friends and all my colleagues to live life to the fullest and continue serving as much as you can. Things can change any minute, any second, and you never know if tomorrow will be the last."

He also remembers "coming together as a nation to help each other out," and says what he lived through that day made him want to continue a life of service and volunteerism supporting the military community. With more than 21 years as a federal employee in health care information technology, he sees his job as "dear to his heart," and his way to continue supporting the military.

On that fateful morning Pagdanganan responded immediately.