There are reports of an unidentified person flying in a jetpack near Los Angeles International Airport (LAX) - the second such incident in two months.
A China Airlines crew said it saw what appeared to be someone in a jetpack on Wednesday at 6,000ft (1,829m), seven miles (11km) north-west of LAX, the Federal Aviation Administration said.
The FBI is investigating the incident, as well as a similar one in September.
It is not clear if either incident posed any danger to aircraft.
A jetpack is a device usually strapped to a person's back, which uses jets of gas or liquid to propel the wearer through the air.
Once the realm of science fiction, they have become a thing of reality in recent years, according to experts.
What is known about the latest incident?
The China Airlines flight reported what it believed to be a person flying in a jetpack at 13:45 local time (20:45 GMT) on Wednesday, the FAA said.
It said it then alerted enforcement agencies, who are now investigating the incident.
"The FBI is in contact with the FAA and is investigating multiple reports of what, according to witnesses, appeared to be an individual in a jetpack near LAX," FBI Los Angeles Field Office spokeswoman Laura Eimiller was quoted as saying by US media.
The airport authorities have so far made no public comment on the issue.
Inventors around the world have constructed jetpacks in the past decade, with one man in Brighton, in the UK, recently breaking the speed record after travelling at a rate of 85.06 mph (136.89km/h).
They have also been used to fly around Dubai and the Statue of Liberty in New York City.