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AT&T said on Wednesday it has ended an experiment that had the company serving ads to those using its free Wi-Fi at two Washington, D.C.-area airports.

“We trialed an advertising program for a limited time in two airports (Dulles and Reagan National) and the trial has ended,” AT&T told Re/code in a statement. “The trial was part of an ongoing effort to explore alternate ways to deliver a free Wi-Fi service that is safe, secure and fast.”

AT&T came under fire this week after computer scientist Jonathan Mayer blogged about his experience encountering the ads while browsing the Web at Dulles International Airport in Washington D.C.

The company didn’t say when the test ended, but it has to be very recently since Mayer said his experience with the ad injections took place last week. Mayer noticed a variety of ads that popped up above the sites he was trying to visit. After doing some digging, he traced them to a start-up, RaGaPa, that pitches its service as a way to help companies monetize their Wi-Fi networks.

AT&T couched the experiment as a way to explore how to pay for the free Internet service. “Our industry is constantly looking to strike a balance between the experience and economics of free Wi-Fi,” it said.

Boingo, which powers the Wi-Fi experience at many airports, typically makes users watch an ad before letting them log on to the Internet for free. It also has a paid, ad-free service, which offers higher speeds.