The runaway balloon incident in Colorado last week was a publicity stunt, police in the state's Larimer County told reporters Saturday.

Police said the parents of a six-year-old boy at the centre of the investigation started the media frenzy about their son being trapped in a free-floating helium balloon to get national attention.

"It has been determined that this was a hoax," Sheriff Jim Alderden said at a news conference in Fort Collins, Colo. "We have evidence to indicate that it was a public stunt done with the hopes of better marketing themselves for a reality television show sometime in the future."

Richard and Mayumi Heene had told authorities they were certain that Falcon, the youngest of their three children, had crawled inside the helium balloon — built in their yard as a science experiment — before it floated away on Thursday.

Television cameras followed the balloon for 80 kilometres before it landed; a search then showed no one inside. A few hours later, Falcon was found safe at home, hiding in the rafters of his family's garage.

Alderden said police decided to interview the parents separately and search their home after watching the family being interviewed by CNN's Wolf Blitzer on the Larry King Live show Thursday night. During questioning, Falcon is heard telling his father that he didn't come out of the attic when called because they were "doing this for the show."

"That's when we had our first 'aha' moment," the sheriff said. "Needless to say, they put on a very good show for us, and we bought it."

The Heene family has been featured twice on the ABC television reality show Wife Swap — the last time in March — fuelling speculation they may have been seeking more publicity.

Alderden said "at some point" his department will be recommending that the local district attorney's office charge the parents with the following offences: conspiracy, contributing to the delinquency of a minor, false reporting to authorities and attempting to influence a public servant.

Richard Heene said he's "seeking counsel," though it was unclear whether he was talking about hiring a lawyer.

"This thing has become so convoluted," Heene told The Associated Press as tears welled up in his eyes.

He said his wife was holding together better than he was.

With files with The Associated Press