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A United statement said "a passenger failed to comply with crew instructions. Local law enforcement officials met the aircraft at the gate and detained the passenger. We will accommodate the remaining customers to Denver on Tuesday morning."
There were 33 passengers and a crew of six on the Boeing 737, according to United Airlines.
"The plane had literally just taken off," said David Cohen, one of the passengers who tackled the man. "We'd just come down the runway and it (plane) started to gain some altitude and I heard some screaming from the back of the plane. A gentleman ran up the aisle and I happened to be sitting in the very first row."
Cohen told ABC News that he saw the man running up the aisle and heard one of the flight attendants yelling "stop him."
"He was yelling 'I got to get to the cockpit -- I'm getting to the cockpit,'" said Cohen.
Cohen said the man was extremely agitated and screaming.
"A couple of us jumped up and I was in the first row, so I tried to jump in that little hallway -- into the galley-way, so he couldn't get to the cockpit. Another gentleman, who was a couple of rows behind me, jumped on him from the back and the two of us kind of wrestled him to the ground," said Cohen
Cohen said he didn't stop think if the plane was in danger, he said he just knew the man needed to be stopped, and that he wanted to make it home.
"Someone had to do something. That's it. He was fighting the whole time, and he was, as I said, extremely agitated and trying to get up. We checked his waistband, we checked his legs and we took his shoes off, just to make sure he didn't have anything on him," said Cohen.
The passenger who was subdued was taken to a Washington hospital, airport spokeswoman Kimberly Gibbs said.
The suspect has not been charged with any crimes and is still under observation.
The delayed flight took off a second time just before 1 a.m. EDT and was scheduled to land at DIA just before 2:30 a.m. MDT.
The swift response of Cohen and others brought comfort to passenger Donna Tellam.
"I think without them it would have been a different story," said Tellam. "It went from a state of the most fear I've ever had in my life, to calm, they had it under control."
Tellam told reporters, that flight attendants had told her they had reservations about the passenger even before takeoff. Tellam says crew members were concerned about the man's behavior, shifting his luggage from different overhead bins and taking pictures inside the aircraft.
7NEWS asked United Airlines about those claims.
"We are reviewing the entire incident," said United Airlines spokesman Luke Punzenberger.