Inside the SkyWest Airlines Plane Where 3 Passengers Passed Out Mid-Flight

— -- SkyWest Airlines and the Federal Aviation Administration appeared to give conflicting explanations for why three people passed out mid-flight Wednesday, forcing a plane to make an emergency landing.

With nearly 80 passengers on board, SkyWest Airlines Flight 5622 -- operating as United Express on behalf of United Airlines -- was 40 minutes into its flight from Chicago to Hartford, Connecticut, when a passenger in the middle of the plane began to lose consciousness.

The pilot feared a potential cabin pressure problem and dropped the plane to get breathable air, according to the FAA.

But the Utah-based regional airline says there are no indications of a pressurization problem on the plane or a problem with the plane's equipment, adding that there were no cockpit warnings and the overhead oxygen masks never dropped.

Marissa Snow, a spokeswoman for SkyWest Inc., told The Associated Press today inspections show "absolutely nothing wrong with the aircraft."

Eric Weiss, spokesman for the National Transportation Safety Board, said, "We're trying to understand the circumstances before we decide on what, if any action, we would take," according to the AP.

The first passenger who lost consciousness received quick attention from Mary Cunningham, a registered nurse sitting nearby who got up to help. Cunningham said the passenger "was gray, her color looked awful."

The nurse said after she got the sick passenger some oxygen, the woman became more alert.

"I went back to my seat," Cunningham told reporters, "and they called me back because the person right behind her passed out.”

But then Cunningham started to feel faint herself. "I started to feel out of breath, so did the flight attendant. Everyone in that section of the flight started to not feel well," Cunningham said.

John Nance, ABC News’ aviation expert, said continued depressurization in a plane, even slightly, could result in passengers’ feeling ill or sick before cockpit alarms go off.

“The tell-tale thing is when the nurse gets up and gets to feel woozy,” he said.

With less oxygen, some passengers who exert themselves will feel the effects more strongly.

With at least three people passed out, the plane began a descent, lowering 27,000 feet in eight minutes, according to SkyWest. The pilot made "an unscheduled landing in Buffalo, New York, in response to a passenger losing consciousness while en route,” the airline said.

Nance said the FAA's explanation makes sense and that the pilot did the right thing, adding the plane "clearly had a cabin altitude problem."

"If you suspect you have a problem, get the plane down to breathable altitude," Nance said. "That's exactly the right thing to do."

After the plane landed, the three people who lost consciousness, as well as 17 others, were evaluated, the AP reported. No one was hospitalized, the airline said in a news release.

The incident is under investigation, but Cunningham, the nurse who witnessed the incident first-hand, said she doesn’t believe this was a medical issue.

"There were multiple people affected," she said.

ABC News' Gillian Mohney and the Associated Press contributed to this story.