A Nest camera in a suburban Denver neighborhood caught the metallic object hitting the ground with such force that it bounced several feet into the air.
Fortunately, no one was hurt from the falling debris.
Meanwhile, passengers on the flight recounted their reactions when the incident happened.
David Delucia was settling back into his airplane seat and starting to relax on his way to a long-awaited vacation when a huge explosion and flash of light interrupted an in-flight announcement and put him in survival mode.
The Boeing 777-200, headed from Denver to Honolulu on Saturday with 231 passengers and 10 crew aboard, suffered a catastrophic failure in its right engine and flames erupted under the wing as the plane began to lose altitude.
As Delucia and his wife prepared for the worst, people in this Denver suburb reacted in horror as huge pieces of the engine casing and chunks of fiberglass rained down on a sports fields and on streets and lawns, just missing one home and crushing a truck. The explosion, visible from the ground, left a trail of black smoke in the sky, and tiny pieces of insulation filled the air like ash.
The plane landed safely at Denver International Airport, and no one on board or on the ground was hurt, authorities said. But both those in the air and on the ground were deeply shaken.
"When it initially happened, I thought we were done. I thought we were going down," said Delucia, who stuffed his wallet in his pocket so he could be easily identified if the plane did go down. "The pilot did an amazing job. It was pretty unnerving."
The Federal Aviation Administration said in a statement that the airplane experienced a right-engine failure shortly after takeoff.
Video posted on Twitter showed the engine fully engulfed in flames as the plane flew through the air. Freeze frames from different video taken by a passenger sitting slightly in front of the engine and posted on Twitter appeared to show a broken fan blade in the engine.
The National Transportation Safety Board is investigating. Authorities have not released any details about what may have caused the failure.
United said in a separate statement that most passengers on United Flight 328 were rebooked on a new flight to Hawaii, but some chose to stay in a hotel overnight instead.
The Broomfield Police Department posted photos on Twitter showing large, circular pieces of debris that appeared to be the engine cowling leaning against a house in the suburb about 16 miles (40 kilometers) northwest of Denver. Police asked anyone injured on the ground to come forward.
Tyler Thal, who lives in the area, told The Associated Press that he was out for a walk with his family when he noticed a large commercial plane flying unusually low and took out his phone to film it.
"While I was looking at it, I saw an explosion and then the cloud of smoke and some debris falling from it. It was just like a speck in the sky, and as I'm watching that, I'm telling my family what I just saw and then we heard the explosion," he said in a phone interview. "The plane just kind of continued on, and we didn't see it after that."
Kirby Klements was inside with his wife when they heard a huge booming sound, he said. A few seconds later, the couple saw a massive piece of debris fly past their window and into the bed of Klements' truck, crushing the cab and pushing the vehicle into the dirt.
He estimated the circular engine cowling at 15 feet (4.5 meters) in diameter. Fine pieces of the fiberglass insulation used in the airplane engine fell from the sky "like ash" for about 10 minutes, he said, and several large chunks of insulation landed in his backyard.
"If it had been 10 feet different, it would have landed right on top of the house," he said in a phone interview with the AP. "And if anyone had been in the truck, they would have been dead."
Based on initial photos and videos posted by passengers, aviation safety experts said the plane appeared to have suffered an uncontained and catastrophic engine failure.
Such an event is extremely rare and happens when huge spinning pieces inside the engine suffer some sort of failure and breach an armored casing around the engine that is designed to contain the damage, said John Cox, an aviation safety expert and retired airline pilot who runs an aviation safety consulting firm called Safety Operating Systems.
"That unbalanced disk has a lot of force in it, and it's spinning at several thousand rotations per minute ... and when you have that much centrifugal force, it has to go somewhere," he said in a phone interview.
Pilots practice how to deal with such an event frequently and would have immediately shut off anything flammable in the engine, including fuel and hydraulic fluid, using a single switch, Cox said.
Former NTSB Chairman Jim Hall called the incident another example of "cracks in our culture in aviation safety (that) need to be addressed."
Hall, who was on the board from 1994 to 2001, has criticized the FAA over the past decade as "drifting toward letting the manufacturers provide the aviation oversight that the public was paying for." That goes especially for Boeing, he said.
The last fatality on a U.S. airline flight involved such an engine failure on a Southwest Airlines flight from New York to Dallas in April 2018. A passenger was killed when the engine disintegrated more than 30,000 feet above Pennsylvania and debris struck the plane, breaking the window next to her seat. She was forced halfway out the window before other passengers pulled her back inside.
In that case, the breakdown was blamed on a broken fan blade in an engine of the Boeing 737. The Federal Aviation Administration ordered airlines to step up inspections of fan blades on certain engines made by CFM International, a joint venture of General Electric and France's Safran S.A.
In 2010, a Qantas Airbus A380 suffered a frightening uncontained engine failure shortly after takeoff from Singapore. Shrapnel from the engine damaged critical systems on the plane, but pilots were able to land safely. The incident was blamed on the faulty manufacturing of a pipe in the Rolls Royce engine.
The Associated Press contributed to this report.