A boy wears U.S. flag-themed socks as riders taking part in the Rolling Thunder First Amendment Demonstration motorcycle run on Memorial Day weekend cross the Memorial Bridge leading from Arlington Cemetery into Washington May 24, 2015. The organizers of the annual event seek to highlight U.S. armed forces members believed to be prisoners of war or missing in action (POW-MIA). REUTERS/Jonathan Ernst

People watch from the back of the Lincoln Memorial as riders take part in the Rolling Thunder First Amendment Demonstration motorcycle run on Memorial Day by crossing the Memorial Bridge leading from Arlington Cemetery into Washington May 24, 2015. REUTERS/Jonathan Ernst

U.S. Air Force veteran James Cloyes of Columbus, Ohio, salutes riders taking part in the Rolling Thunder First Amendment Demonstration motorcycle run on Memorial Day by crossing the Memorial Bridge leading from Arlington Cemetery into Washington May 24, 2015. REUTERS/Jonathan Ernst

Connie Miller of Woodbridge, Virginia, holds a Thank You sign as riders take part in the Rolling Thunder First Amendment Demonstration motorcycle run on Memorial Day by crossing the Memorial Bridge leading from Arlington Cemetery into Washington May 24, 2015. REUTERS/Jonathan Ernst

Riders take part in the Rolling Thunder First Amendment Demonstration motorcycle run on Memorial Day by crossing the Memorial Bridge leading from Arlington Cemetery into Washington May 24, 2015. REUTERS/Jonathan Ernst

Motorcyclists riders make a crayon rubbing of a name on the Vietnam Veterans Memorial wall on Memorial Day weekend in Washington May 24, 2015. REUTERS/Jonathan Ernst

Riders take part in the Rolling Thunder First Amendment Demonstration motorcycle run on Memorial Day by crossing the Memorial Bridge leading from Arlington Cemetery into Washington May 24, 2015. REUTERS/Jonathan Ernst

People look on with signs as riders take part in the Rolling Thunder First Amendment Demonstration motorcycle run on Memorial Day by crossing the Memorial Bridge leading from Arlington Cemetery into Washington May 24, 2015. REUTERS/Jonathan Ernst

A woman holds a U.S. flag as riders taking part in the Rolling Thunder First Amendment Demonstration motorcycle run on Memorial Day weekend cross the Memorial Bridge leading from Arlington Cemetery into Washington May 24, 2015. REUTERS/Jonathan Ernst

Boy Scouts Michael Beene (L) and his brother Patrick Beene (3rd L) join U.S. Air Force veteran James Cloyes to salute riders taking part in the Rolling Thunder First Amendment Demonstration motorcycle run on Memorial Day by crossing the Memorial Bridge leading from Arlington Cemetery into Washington May 24, 2015. REUTERS/Jonathan Ernst

Riders take part in the Rolling Thunder First Amendment Demonstration motorcycle run on Memorial Day by riding around the National Mall in Washington May 24, 2015. REUTERS/Jonathan Ernst

Boy Scouts Michael Beene (L) and his brother Patrick Beene (3rd L) join U.S. Air Force veteran James Cloyes to salute riders taking part in the Rolling Thunder First Amendment Demonstration motorcycle run on Memorial Day by crossing the Memorial Bridge leading from Arlington Cemetery into Washington May 24, 2015. REUTERS/Jonathan Ernst

U.S. Air Force veteran James Cloyes of Columbus, Ohio, salutes riders taking part in the Rolling Thunder First Amendment Demonstration motorcycle run on Memorial Day by crossing the Memorial Bridge leading from Arlington Cemetery into Washington May 24, 2015. REUTERS/Jonathan Ernst

Onlookers wave flags as riders take part in the Rolling Thunder First Amendment Demonstration motorcycle run on Memorial Day by crossing the Memorial Bridge leading from Arlington Cemetery into Washington May 24, 2015. REUTERS/Jonathan Ernst

People visit the Vietnam Veterans Memorial wall on Memorial Day weekend in Washington May 24, 2015. REUTERS/Jonathan Ernst