A six-year-old boy in the US feared to have floated off in a helium balloon was found safe at his home, hiding in a cardboard box in the garage attic, police said last night.

Sheriff Jim Alderman turned to reporters during a news conference in Colorado, held his thumbs up and said: "He's at the house."

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The sheriff said an investigator on the scene saw the boy and he was fine.

He said the boy had apparently been in the attic the whole time.

The boy's brother had said he saw Falcon Heene get in the balloon before it took off.

The flight lasted two hours and spanned 50 miles, setting off a frantic search.

The bizarre scene was played out live on television as the balloon rotated slowly in the wind, tipping precariously at times before gliding to the ground.

Sheriff's officials said from the beginning that Falcon was in the balloon and authorities feverishly searched for any sign of the child on the ground.

Cathy Davis, of the Larimer County Sheriff's Department, told reporters that the balloon, which was in the air for two hours, was owned by the boy's parents, Richard and Mayumi Heene, and tethered behind the family's home.

She said their two sons were playing outside when the older boy saw Falcon go into a compartment at the bottom of the balloon and fly away.

Mr and Mrs Heene are storm chasers and have also appeared on the ABC reality show Wife Swap.

The Colorado Army National Guard sent an OH-58 Kiowa helicopter and was preparing to send a Black Hawk UH-60 to try to rescue Falcon, possibly by lowering someone to the balloon.

They were also working with pilots of ultralight aircraft on the possibility of putting weights on the home-made craft to push it down.

But the balloon landed on its own in a field and sheriff's deputies secured it to keep it in place, even tossing shovelfuls of dirt on one edge.

Northbound departures at Denver International Airport were suspended as a precaution against a possible collision between the balloon and an aircraft.

Federal Aviation Administration spokeswoman Laura Brown said the agency tracked the balloon through reports from pilots.

Neighbour Lisa Eklund saw the balloon taking off.

"We were sitting eating, out looking where they normally shoot off hot-air balloons. My husband said he saw something. It went over our rooftop. Then we saw the big round balloonish thing, it was spinning," she said.

"By the time I saw it, it travelled pretty fast."

The story gripped the television news networks, which set aside other programming to follow the balloon and speculate on the safety of the boy.

It was not clear why the boy's brother reported seeing Falcon getting into the balloon.

Kevin Kuretich, of the Colorado Division of Emergency Management, said the craft had some kind of electric power unit which was run by double-C batteries.

Jason Humbert, who saw the balloon land 12 miles north east of Denver International Airport, said: "It looked like an alien spaceship you see in those old, old movies."

Neighbour Bob Licko, 65, said he was leaving home when he heard a commotion in the Heenes' back garden. He said he saw two boys on the roof with a camera, commenting about their brother.

"One of the boys yelled to me that his brother was way up in the air," Mr Licko said.

He said the boy's mother seemed distraught and the boy's father was running around the house. The Poudre School District in Fort Collins, where the boys attend, had no elementary school classes today because of a teacher work day.