WASHINGTON — Three years ago, AT&T warned smartphone customers with “unlimited” data plans that their connections might be slowed if they used a lot of data. On Tuesday, the Federal Trade Commission said AT&T’s disclosure was deceptive because it was not specific enough.

The commission filed a federal lawsuit against AT&T on Tuesday, saying the company had misled customers by slowing the connections of people with unlimited plans after they used more than two gigabytes of data in a month.

For some of the people who hit that threshold, the F.T.C. said, downloads were slowed by as much as 95 percent, essentially making their smartphones unable to gain access to the Internet or use certain apps.

“AT&T promised its customers unlimited data, and in many instances it has failed to deliver on that promise,” said Edith Ramirez, chairwoman of the agency. “The issue here is simple: ‘Unlimited’ means unlimited.” The commission, which does not have the power to impose fines, said it would seek “millions of dollars” in restitution for consumers.