OTTAWA, Ill. >> Falling at speeds of up to 220 mph, nearly 140 skydivers shattered the vertical skydiving world record as they flew heads-down in a massive snowflake formation in northern Illinois.

Three judges representing the Federation Aeronautique Internationale, the international air sports agency, certified that 138 skydivers created the formation Friday evening over Ottawa, about 80 miles southwest of Chicago. It took 15 attempts over three days for the team to break the previous record of 108 skydivers, which was set in 2009.

“I feel amazing,” Rook Nelson, an organizer and the owner of Skydive Chicago where the record was broken, said shortly after he made the jump. “There was a lot of emotion and a lot of days where we should have got it. But we dug down deep and stuck at it.”

Following months of planning, tryouts and camps to decide who could take part in the dangerous challenge, the record breakers squeezed into six aircraft and launched themselves into the air at 18,500 feet. Flying at such a high altitude presents a risk of altitude sickness, so the skydivers and pilots used oxygen tanks aboard the planes.

Four camera operators shooting video and stills jumped with the 138 participants to record their achievement for the FAI judges. Those images are key, said co-organizer Mike Swanson, a professional skydiver who base jumped from Willis Tower and its fellow Chicago skyscraper Trump Tower for the movie “Transformers 3.”

If no one records the 150-foot-wide formation showing all the jumpers in their pre-assigned slots, “then it wasn’t really done,” Swanson said.

The challenge for the record began Wednesday, midway through a 10-day skydiving festival. After each attempt, the organizers reviewed the videos and decided who should stay and who should be swapped out for one of the dozens of hopefuls who didn’t make the initial cut.

“It’s a hard job coming in from the bench,” said Erica Tadokoro, from Brisbane, Australia. “You have to be positive because it’s a team effort.”

Tadokoro, 43, was one of just 13 women selected in the first string. She was cut after the 14th unsuccessful attempt — one shy of the record jump.

Nelson explained that vertical flying is “basically doing a headstand” in the air. The lack of wind resistance speeds the skydivers’ fall rate to an average of 170 to 180 mph. Ahead of the record attempt, he said some of those involved would need to reach much higher speeds. And that increased the risks.

If they’re not paying attention when diving into the formation at upwards of 220 mph, “it’s going to be like someone running a red light and you taking them out,” Nelson said.

Each skydiver knew exactly when to exit the aircraft, whom to follow and where in the formation they should be. At 7,000 feet, the skydivers began to peel away on a last-in, first-out basis, and each wave deployed their parachutes at altitudes specified according to their positions in the formation.

“We don’t want everyone to open their parachutes at the same altitude because then everybody lands at the same time. We stack the sky vertically” to avoid collisions, Nelson said.

The United States Parachute Association says that of an estimated 3 million skydives made across the U.S. in 2011, 21 were fatal. And most skydiving accidents happen under canopy.

Skydivers traveled from all over the world to take part in the record attempt, including from France, Spain, Norway, Sweden, Switzerland, Ukraine, Russia, Italy, Belgium, Australia and the U.K. And one, 40-year-old Ahmed Sferi from Reunion, said he traveled for two days from his tiny Indian Ocean island home to reach Chicago so he could take part.