In the "Luv a Fare Sale" in January, Southwest offered one-way nonstop flights for less than $100. But the agency says the deal was offered for only 1% or 2% of the seats available on some routes.

In another deal offered in January, the federal agency said Southwest offered $66 flights from Dallas to Branson, Mo. But the DOT said it found no seats available for that price during the sale period.

In a statement to the DOT, Southwest said none of the $66 fares were available due to a technical glitch.