By JOE MANDAK, Associated Press

SHANKSVILLE, Pa. (AP) — Vice President Mike Pence told relatives and friends of the victims of United Flight 93 attending the 16th anniversary ceremony Monday in Pennsylvania that the passengers might well have saved his life.

The somber service began at 9:45 a.m., the time that federal investigators determined passengers decided to revolt against their four al-Qaida hijackers, who ended up crashing the plane in a field 60 miles (100 kilometers) southeast of Pittsburgh.

The Republican said he was in Washington as a member of Congress on 9/11, and learned that the Capitol was a possible target of the hijacked plane, which was only 12 minutes away.

"It was the longest 12 minutes of my life," he said, adding they soon learned the plane went down in a field in Pennsylvania.

Thirty-three passengers and seven crew members on the flight from Newark, New Jersey, to San Francisco were killed.

Photos: Remembering 9/11 View All 16 Images

"Without regard to personal safety, they rushed forward to save lives," he said. "I will always believe that I and many others in our nation's capital were able to go home that day and hug our families because of the courage and sacrifice of the heroes of Flight 93."

About 1,000 spectators, including dignitaries and the victims' friends and families, gathered on a breezy, cool and cloudless morning as bells were tolled and the victims' names were read. Pennsylvania Gov. Tom Wolf, a Democrat, and U.S. Interior Secretary Ryan Zinke spoke before Pence.

The ceremony marked the beginning of the end of a $46 million effort to transform the rural Pennsylvania crash site into a national memorial park.

Ground was broken Sunday on the final element of the Flight 93 National Memorial — a 93-foot (28.4-meter) tall Tower of Voices. The tower, to be built near the park's entrance, will feature 40 tubular metal wind chimes, one each for the victims.

Bill Adderly, the father of flight attendant CeeCee Ross Lyles, said the Tower of Voices is an apt ending to the memorial site.

"We were here two days after the crash, and never in our imagination did we believe it would be this beautiful," said Adderly, of Fort Pierce, Florida.

Adderly and his wife, Shirley, were among the family and friends of the victims present for Sunday's tower groundbreaking. At the ceremony, officials played a simulation of what the 40 tubular chimes will sound like when the wind blows through them at the mountaintop memorial.

"The bells, the ringing, I could hear our daughter's voice in it. She loved to sing. She loved to praise the Lord."

Actual construction on the tower will begin later this month. It should be completed in time to dedicate the tower for the 17th anniversary ceremony next year.