Bradley International Airport staff is trying to find out why two of three power sources failed Monday morning, causing on-and-off outages, which delayed or canceled flights and led police to block the access road. 

The outages started happening intermittently before dawn, sometime before 4:45 a.m. The airport was closed to arriving cars for hours, forcing people to haul their luggage more than a quarter of a mile up a hill to the terminal. The access road was reopened by 8:50 a.m.

The airport had problems with both its cogeneration plant and its direct feed from the power company, Eversource, said Alisa Sisic, airport spokeswoman, early Monday afternoon. A third source of power, a generator, functioned properly, but it does not have the capacity to power the entire airport.

“At this time, we are fully processing passengers,” she said shortly after noon. The airport continues to evaluate each component of its power systems, she added.

“We apologize for the inconvenience, and we thank our passengers for their patience and understanding,” Sisic said.

TSA screening had resumed by 7:30 a.m., but it was too late for some. Many flights were delayed and others were canceled.

Two outbound Southwest Airlines flights — one to Chicago and one to Baltimore — were among those canceled. Although lines remained long at airline ticket counters at 9:30 a.m., TSA lines seemed to be moving and flights had begun to depart.

There was a large police presence, which included state troopers and police dogs, and the terminal remained packed with delayed travelers.

One of them was Mike LaBonte of Granby, Mass. He said he had left his home at 3 a.m. and got to the airport around 4. He said he’ll be waiting all day to get his rescheduled Southwest flight to Tampa around 7 p.m.

“We already knew from the driver of the parking service that the airport had lost power,” LaBonte said. “I was really surprised that they had lost power around 3 a.m. but the first email I got from the airline was at 5:25, five minutes after my flight was supposed to leave.”

He said he got into the terminal to wait sometime after 5, but there was no power.

“The lights kept coming and going. They’d go on then off, on then off,” he said.

Nicci Test, a Connecticut native who was visiting her family in Stafford for a long weekend, said she was one of the many who had to lug her bags up the hill of the terminal access road.

“Walking up that hill, it’s cold when you’re not expecting it,” she said. “I didn’t even know there were any delays until after I left for the airport.”

Airport staff is working with the power company to find out what happened, the airport said.

“We are working closely with Eversource to determine the cause of the power loss and to restore full power as quickly as possible. Our staff is working as quickly as possible to return operations back to normal with our TSA and airline partners,” the airport said.

Eversource doesn’t know what caused the outages, its spokesman said shortly before 9:30 a.m.

“Job no. 1 is getting things fixed and getting things back to normal up there,” Mitch Gross said.

The incident may change some fliers’ minds about Bradley, which significantly improved its standing among medium-sized, U.S. airport peers, according to a September customer satisfaction survey.

Some travelers complained on Twitter that the airport did a poor job communicating about the incident. Another said she was kicked off a flight after waiting outside for five hours.

By 10:30 a.m., the airport had quieted down significantly. The TSA had no lines.

Courant Staff Writer Nicholas Rondinone and Photographer Patrick Raycraft contributed to this report.