The flight, carrying 75 passengers, departed from O'Hare International Airport in Chicago and had been bound for Bradley International Airport in Windsor Locks, Connecticut. The plane began a steep, rapid descent after the crew declared an emergency and landed at Buffalo Niagara International Airport in Buffalo at about 11:40 a.m.
Federal transportation safety authorities said Thursday they are still assessing what happened on Flight 5622, an Embraer E170 jet operating as United Express.
Passenger Larry Johnson of Danbury, Connecticut, told The Associated Press it became difficult to breathe part way through the flight, though oxygen masks never dropped.
"They told us there was a leak in something and the pressurization was cutting short," he said. "They said if you got lightheaded, that was normal, but that we were going to have to descend and make an emergency landing."
He said several passengers were given oxygen.
"None of the air vents were working and it was hard to breathe. You just felt that your chest was caving in and then the plane descended so rapidly and that didn't help," Johnson said. "Me and my girlfriend, we were looking at each other we were like, 'We don't feel good.' Everything was so bright and when you blinked you would see dots. It was just hard to breathe. It wasn't a normal thing."
But SkyWest Inc. spokeswoman Marissa Snow said Thursday that inspections of the aircraft's systems by the airline's mechanics and local authorities "shows absolutely nothing wrong with the aircraft."
She said she had no confirmation no air was coming from the vents in the cabin or that there was a problem with the air handling system in the cabin.
"It is very unusual, obviously, to have a loss of consciousness," she said, adding that "there's no issue with the airplane."
The plane remained at the Buffalo airport Thursday, Snow said. It eventually will be flown to an airfield where the St. George, Utah, company has a facility, she said.
Initial reports the cabin depressurized turned out to be wrong, authorities said. It is standard procedure for pilots to begin a rapid descent to 10,000 feet if they believe an aircraft has a pressurization problem.
"We're trying to understand the circumstances before we decide on what, if any action, we would take," said Eric Weiss, a National Transportation Safety Board spokesman, said Thursday. The Federal Aviation Administration said it is investigating, but had no new information.
Mary Cunningham, a nurse at Yale-New Haven Hospital in Connecticut was on the flight. She told WTNH-TV she helped provide medical attention after the first woman passed out.
"We got her oxygen and as soon as she got on the oxygen she was alert, she came right back," Cunningham said. "Then I went back to my seat after she was feeling better and they called me right back because the person sitting right behind her passed out."
Johnson said that "everyone in the middle of the cabin basically felt like they couldn't breathe and that's when they knew something was wrong."
For nearly eight minutes, the plane descended at a very steep incline, dropping as fast as 7,000 feet per minute, flight tracking service FlightAware said.
In addition to the three passengers who lost consciousness, an additional 15 adults and two children were evaluated upon landing. None required treatment outside the airport, airport spokesman C. Douglas Hartmayer said.
"Anyone who said they didn't feel well was treated at the gate," he said.
Marc Moller, an aviation lawyer in New York, said that passenger accounts suggested a problem with the aircraft. The rapid descent also could have caused panic and additional discomfort for passengers, even if pressure in the cabin was normal, he said.
"You expect to go from point A to point B without having trouble breathing," Moller said. "If you have trouble breathing as these passengers describe, something went wrong and the airline is responsible."
Moller said SkyWest's liability would be less if the passengers recover fully from having lost consciousness.
Associated Press writers Chris Carola in Albany, New York, and David Koenig in Dallas contributed to this report.
Copyright 2015 The Associated Press modified.