WASHINGTON (Reuters) - The rumble of motorcycle engines filled a rainy morning in Washington on Sunday as thousands of bikers paraded around the National Mall for the 30th annual Rolling Thunder rally to raise awareness of U.S. military personnel who went missing in action.

Participants, many of them U.S. war veterans astride the loud, muscular Harley-Davidson machines typical of such events, rode into Washington led by a police escort over the Memorial Bridge, where cheering spectators waved Americans flags in support.

“The greatest thing a person can ever do for a veteran is say: ‘Thank you,’ and this is just one giant thank you,” said Jason White, who fought in the 1990-1991 Gulf War.

The Rolling Thunder “Ride for Freedom” began in 1987 as a protest demanding a full accounting for American troops listed as missing in action, some who were believed at the time to still be held as prisoners following the Vietnam War.

The annual Memorial Day event has evolved to encompass efforts to honor and raise awareness about the fate of all U.S. service members who were lost or otherwise “abandoned” following any of America’s military conflicts.

According to U.S. Defense Department figures, nearly 83,000 American military personnel remain unaccounted for, the overwhelming majority, 73,000-plus, from World War Two. The tally also includes more than 7,700 from the Korean War and over 1,600 from the Vietnam War.