All United Airlines flights in the US were grounded this morning for nearly an hour, over "dispatching information."

Though United hasn't indicated the cause for the grounding, tweets from onboard passengers and a news report have indicated various possible reasons.

"United began delaying flights at approximately 8 a.m. CT to ensure aircraft departed with proper dispatching information," United spokesman Charles Hobart wrote an in email to WIRED. "We resumed departures at about 8:40 CT and are accommodating our customers to their destinations."

Hobarth didn't explain what "dispatching information" meant, but passenger Edward Benson, the founder and CTO of the tech firm Cloudstitch, sent out a tweet indicating that his pilot had told passengers on his plane that they were being grounded due to a possible hack of United's computer network, which resulted in bogus flight plans popping up in the system.

He followed that tweet with another one about an hour later after the problem appeared to have been resolved.

Other passengers tweeted similar messages about fake flight plans being the cause.

United's spokesperson Hobarth, in a follow-up call late Tuesday, appeared to dispute the idea that a hacker was involved. "What we found is that there's no indication that this was caused by an outside entity," he said. Hobarth declined to address whether the dispatch issue involved erroneous flight plans.

United Airlines has been embroiled in a controversy recently over claims that planes it and other airlines operates can be hacked.

In April, the airline had a security researcher named Chris Roberts removed from a flight after he sent out a tweet joking about vulnerabilities in the United plane, a Boeing aircraft, that he was flying from Denver to Chicago. United Airlines contacted the FBI, who removed Roberts from a second plane after he landed in New York, interrogated him for several hours and seizing his electronic equipment.

Roberts has been studying the security of Boeing and Airbus planes since 2009 and told the airlines that he was able to jump from their inflight entertainment systems and their SATCOM systems to the plane avionics networks after plugging his laptop into a network box beneath passenger seats.

The FBI indicated in an affidavit that the Roberts had told the FBI he had hacked several planes while inflight and had been able on one occasion to briefly commandeer a plane. Both Boeing and Airbus, however, dispute that their planes can be hacked by onboard passengers. Aviation experts also told WIRED that the hack described in the FBI's affidavit is impossible.

Last month, United Airlines announced it was launching a bug bounty program to reward security researchers who uncover flaws in its websites, apps and online portals. The bounty program, however, does not cover the most crucial vulnerabilities researchers could find—those discovered in onboard computer networks, such as the Wi-Fi and entertainment systems. The bounty program specifically excludes “bugs on onboard Wi-Fi, entertainment systems or avionics” and United notes that “[a]ny testing on aircraft or aircraft systems such as inflight entertainment or inflight Wi-Fi” could result in a criminal investigation.