Frontier Airlines and five foreign carriers have agreed to refund more than $600 million combined to travelers whose trips were canceled or significantly delayed since the start of the pandemic, federal officials said Monday.

The U.S. Department of Transportation said it also fined the same airlines more than $7 million for delaying refunds so long that they violated consumer-protection rules.

The largest U.S. airlines, which accounted for the bulk of complaints about refunds, avoided fines, and an official said no other U.S. carriers are being investigated for potential fines.

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That did not satisfy consumer advocates, who said that the major U.S. airlines also violated rules around refunds — even if they took corrective steps more quickly.

“Frontier was a bad player in all this, and they deserve to be fined, and we’re glad they are paying the refunds they were supposed to pay, but we are very critical of how the DOT just seems to not want to go after the biggest fish, the ones causing the most problems,” said Bill McGee of the American Economic Liberties Project, a non-partisan group that opposes concentrated industrial power.

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