Travellers aboard a US flight tackled a fellow passenger to the ground after he ran down the aisle towards the cockpit screaming about "jihad".
Grainy video of the incident appeared on social media, with the man pinned to the floor of the aircraft while repeatedly mumbling "I'm sorry, I'm sorry".
United Airlines Flight 1074 was en route from Washington to Denver when the incident occurred.
The pilots were forced to place an emergency alert call before making a U-turn and head back to Washington Dulles airport while the other passengers held the man down.
Photographs and video show the dark-haired man bruised, blooded and mumbling.
In the cellphone video, another passenger is heard saying: "Don't move. You’re okay, We're going to get you off this plane, buddy."
“We are declaring an emergency due to a passenger disturbance,” the pilot told air-traffic controllers.
A spokesman for Washington's airport authority said the passenger was taken to a hospital for observation. The man has not been charged and Dulles' operations were unaffected, he said.
The Boeing 737 plane, which was carrying 33 passengers and six crew members, had left Dulles for Denver on Monday evening.
Passenger Donna Tellam told ABC News, "At one point when his head was down he said there were jihadists in the cargo hold and he did say jihad a couple times."
She said the man was held down by passengers until flight attendants put plastic restraints on his arms, according to a Reuters report.
CNN played audio from LiveATC.net, which provides air traffic control communications online, of the pilot's radio traffic with controllers.
"We're declaring an emergency due to a passenger disturbance," the pilot said.
"He ran forward towards the cockpit and he is being restrained by other passengers ... The cockpit is secure and we would just like to return to the airport and have authorities meet him."
The man never reached the cockpit, which remained securely locked throughout the incident.
The man was removed from the plane after it landed back at Dulles at 10.30pm and taken to a hospital for evaluation. No gun or weapon was found, Reuters said.
The airline said that the other passengers would be flown to Denver on Tuesday.