Airlines are pushing back on sharing their information with fare comparison sites like Kayak and Orbitz, Sen. Charles Schumer said Sunday as he called for a federal investigation into whether air carriers are colluding with each other.

"The idea of comparison shopping is running into a lot of turbulence," Schumer said.

Bargain sites like TripAdvisor save passengers over $6 billion a year, but officials say several airlines are beginning to withhold their schedule data and price information from the smaller travel sites.

Lufthasana will also start charging a fee this fall for any reservation booked through a comparison shopping site online or a travel agent.

"Until now, the percentage of revenue generated from the sale of flight tickets by our airlines has continuously decreased," Lufthansa said in a statement last month. "While other service and system partners in the value chain are recording increasing margins and returns, our airline's earnings have been compromised over time, even though they are the actual providers of flight services. We want to counteract this trend by refocusing our commercial strategy."

"If the airlines are colluding, it's illegal to do it," Schumer said. "It doesn't seem likely they are doing it alone -- because if only one did it, they would lose out."

He added that the Department of Justice and the Department of Transportation are already probing why fares have stayed high as fuel prices have dropped.