Per interview with Aaron, he seemed to have seen a flash of a gun, but sometimes details do get distorted and exaggerated:
"Aaron remembers seeing the metal flash of a handgun and hearing a voice break the darkness.
A man who seemed to have come out of nowhere told them to stop, he has a gun, get off their bikes and lay down in the ditch on their stomachs."
http://www.parkrapidsenterprise.com/content/wetterling-friend-relives-kidnapping-daily
the article from the Strib only says "flash of a gun":
By Richard Meryhew, Star Tribune
October 21, 2009
Even now, 20 years later, Aaron Larson plays it back daily, wondering what he might have done, said or seen to make things turn out differently. "It's difficult to think about all the things that might have been," Larson said softly. "It's something I think about every single day."
"The first thing I remember was the flash of the gun, and a guy saying, 'Stop, I have a gun,'" Larson recalled. "I caught my breath. I thought it was a high school kid pulling a joke on us. ... Then it hits you: this is happening, it's no joke." The man ordered them to lie face-down in the roadside ditch. Larson remembers his heart "going 1,000 beats a minute," but having no clue what was happening. "You didn't hear about people being kidnapped or abducted. It didn't cross my mind." The man asked Trevor to look at him, then asked his age. He did the same with Aaron, then Jacob. "Then he told Trevor to run as fast as he can to the woods...