Six-year-old Falcon Heene had been reported to be aboard a flying-saucer-shaped balloon fashioned by his father and then carried by high winds on to the plains of eastern Colorado, but later said he was hiding in the garage. (David Zalubowski/Associated Press)

Authorities in Colorado say any charges against the couple accused of masterminding the runaway balloon publicity stunt won't come until at least next week.

Eloise Campanella, a spokeswoman for the Larimer County sheriff's office, said Monday that investigators likely won't finish their reports and present them to the district attorney's office until next week. It will then be up to prosecutors to decide whether to file charges against Richard Heene or his wife, Mayumi.

The couple's lawyer, David Lane, had expected charges to be laid by Wednesday.

Lane said Richard Heene and his wife, Mayumi, are willing to turn themselves in if and when charges are laid, but declined to say whether he believed they were involved in what police described as a hoax.

"If they [prosecutors] can prove their case beyond a reasonable doubt, that's one thing. If they can't prove their case beyond a reasonable doubt, that's another," the defence attorney told The Associated Press.

The Heene family began a media frenzy last week when the parents told authorities they were certain that Falcon, the youngest of their three children, had crawled inside a 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.

When interviewed after the alleged incident by CNN's Wolf Blitzer on the Larry King Live show Thursday night, Falcon told his father that he didn't come out of the attic when called because they were "doing this for the show."

Police said they believe the incident was a hoax after interviewing the family members separately. Larimer County Sheriff Jim Alderden said Sunday that he would be seeking charges, including felonies, against the parents, who had twice appeared on the reality show Wife Swap.

"We certainly know that there's a conspiracy between the husband and wife. You've probably seen some of the emails and some of the things on the internet suggesting that there may be other conspirators," said Alderden.

Media stunt in works for months: report

Investigators had said Sunday they would also be questioning Robert Thomas, a 25-year-old Denver man who claimed Richard Heene had discussed with him a media stunt involving a weather balloon to help launch a proposed reality show.

Thomas sold his story to entertainment gossip website Gawker.com and provided the website with email exchanges between himself and Heene.

Thomas said Heene envisioned himself as the star of a reality show in which he would carry out scientific experiments and hunt for signs of extraterrestrial life.

"This will be the most significant UFO-related news event to take place since the Roswell Crash of 1947, and the result will be a dramatic increase in local and national awareness about the Heene family, our reality series, as well as the UFO phenomenon in general," according to a copy of the show's proposal provided to the site by Thomas.

Gawker editor-in-chief Gabriel Snyder said Thomas was planning to meet with investigators Sunday night, though authorities did not confirm this.

Alderden said the children were still with the parents Sunday and that child protective services had been contacted to investigate their well-being.

The sheriff said he expected to recommend charges of conspiracy, making a false report to authorities, attempting to influence a public servant and contributing to the delinquency of a minor.

Kids' behaviour under scrutiny

Alderden said on Sunday the behaviour of the children during interviews raised questions whether they were given drugs beforehand. The three boys appeared dazed and Falcon threw up twice during interviews his family gave on Friday.

Police also said documents show the Heenes have agreed to sell their story to a media outlet, though Snyder said it was not his publication.

The producer of Wife Swap said it had a show in development with the Heenes, but that the deal was now off. TLC also said Heene had pitched a reality show to the network months ago, but that it had passed on the offer.

With files from The Associated Press