A power outage at Ronald Reagan Washington National Airport -- the third outage this year -- delayed flights, closedsecurity checkpoints and generally gummed up the works Thursday for hundreds of travelers.

Ticketing and X-ray machines at the airport's busy B and C terminals blinked out around 11:45 a.m., making it difficult for airline officials to get passengers through to their gates.

Departure boards indicated about one in five flights were delayed or canceled by early afternoon, as airport officials scrambled to identify the source of the power failure.

"There was a fire at an electrical substation near our headquarters, but we don't know the cause of the fire yet," said Courtney Mickalonis, a spokeswoman for the Metropolitan Washington Airports Authority. Mickalonis said the fire was contained to one small electrical room.

Airport officials late Thursday had yet to determine how the fire began.

Reagan National lost power for roughly an hour earlier this month, the result of a planned systems check that went awry. The airport also lost power in January when an airport substation suffered a technical difficulty, and experienced flooding in one terminal earlier this year.

Hundreds of weary travelers were stuck in long lines near the Delta, Continental and US Airways ticketing counters on Thursday.

"This couldn't have happened at a worse time," said Jordan Blaydes, 33, a Dallas native.

Blaydes was trapped in line with his bags outside of the Delta counters shortly after he departed Microsoft's worldwide partner conference.

Airport officials were able to restore power to the terminals around 2 p.m., but Mickalonis said the electricity was still out at the airport's headquarters late into the afternoon.

She said it would take at least the reminder of the day for ticketing traffic to return to normal.

Travelers were not happy.

"We were planning to see more sights," said Melanie Kelly, 47, who was in town for a vacation with her son, Stephen.