Mike Robinson was on the step outside the plane, 12,000 feet above ground, just seconds from making the 937th sky dive of his career, his final jump of the day.

Then came “that terribly loud bang,” he recalled Sunday.

The wing and its supporting strut, just a few feet from Robinson and three other jumpers, exploded into a fireball over northwestern Wisconsin, 8 miles southeast of downtown Duluth, early Saturday evening. A second plane, carrying five other sky divers, had collided with Robinson’s plane, sending the jumpers into a premature free fall.

What followed, said Robinson, an experienced instructor and safety adviser for Skydive Superior LLC, was “nothing short of miraculous.”

Authorities on Sunday still didn’t know what caused the accident. The lead plane broke into three pieces and one of its wings had yet to be located Sunday afternoon, said Roland Herwig, a spokesman for the Federal Aviation Administration. But as investigators from the FAA met with officials from Duluth and Superior, Robinson, 64, spoke of falling from the sky while dodging debris as nine jumpers and two pilots landed safely, with little more than a few cuts and bruises. One pilot managed to guide his plane safely to the runway, while the other bailed out of his doomed plane using an emergency parachute.

“We call it our sunset load, the last sky-dive of the day,” Robinson said. “The sun was getting low on the horizon, still daylight. Beautiful blue skies. Just a perfect setting to make the last dive of the day.”

On board with Robinson in the lead plane were Barry Sinex, 54, of Duluth; LaNaya Bonogofsky, 31, of Superior, and Johnny Rodorigo, 26, of Duluth, all experienced sky divers, individuals who had each jumped between 300 and 2,500 times, Robinson said. All are passionate about their sport, Robinson said, and Rodorigo agreed. Two years ago, Sinex even wrote an essay for Parachutist Online entitled, “How Skydiving Changed My Life.”

Saturday’s experience could have been life-ending, Robinson said.

The two planes left Richard I. Bong Airport in Superior about 5:45 p.m. and were in the air for 15 minutes when the nine parachutists prepared to jump. Robinson said he could see three sky divers on the step of the trailing plane, with two others still inside.

“We were just a few seconds away from having a normal sky-dive when the trail plane came over the top of the lead aircraft and came down on top of us,” Robinson said. “There was this terribly loud bang. And a flash of fire. The wing and the strut supporting it broke off into a fireball, leaving us nothing to hang on to.

“Even if anybody yelled anything, it was too loud to hear anything. We followed our instincts. We just jumped. And we watched everything falling around us.”

There was no time to contemplate what had happened. Blake Wedan of Duluth, the experienced pilot of the trailing plane, is highly regarded, Robinson said. But Robinson and his fellow jumpers were likely thinking of only self-preservation for the next 60 to 90 seconds.

“Our concern was not to get hit by anything,” he recalled. “We were in a free fall. By this time, the other wing had come off. Both wings were above us. All we could hope for, at that point, was to fall faster than those wings and somehow get away from them.

“At that point, we’re trying to save our own lives.”

Between 3,000 and 5,000 feet above ground, the sky divers opened their chutes, guiding them like gliders for the next four minutes. While the four parachutists from the lead plane landed in their normal landing area — with emergency crews racing to meet them — there was concern for their Twin Cities-based pilot, a guy nicknamed “Tweak,” who had an old military-style emergency chute. He landed safely in a grassy area, away from the others, and was later hospitalized with minor injuries and a gash, Robinson said.

More amazing was the landing of the other pilot, Wedan. Despite a damaged propeller, Wedan landed his Cessna 185 safely at the Superior airport.

“You could re-create this thousands of times and I don’t think you could come out with a more positive outcome,” Robinson said.

Robinson, who says he grew up in the Twin Cities and worked for years for the Minnesota Department of Transportation, said he turned to sky diving in 2001 and “never looked back.”

Some of the sky divers weren’t immediately comfortable talking about Saturday’s experience, Rodorigo said. But there was talk Sunday of flying occupants of both planes to New York to share their story on national television.

Saturday’s incident left several of the jumpers in “rough emotional states,” but Robinson said he would be surprised if any stopped sky diving.

“Right now, we don’t have any planes [at Skydive Superior],” he said. “One’s damaged, the other destroyed.

“But I’ll have no hesitation to get back up there.”