Top GOP lawmakers on the House Energy & Commerce Committee Jan. 12 sent a letter to Apple Inc. expressing concern over the company’s throttling of older iPhones, after the company briefed committee staff on the issue.

The letter spotlights the increasing political pressure facing Apple after the company acknowledged in December it had slowed the performance of older iPhones, including the iPhone 6 and iPhone SE, to prevent aging batteries from unexpectedly shutting down. The company Dec. 28 apologized and offered affected customers reduced-priced batteries through 2018.

Apple told committee staff it couldn’t be sure it has “taken the necessary steps” to ensure customers of the affected phones can get replacement batteries in a timely manner, committee chairman Greg Walden (R-Ore.) and three other Republicans wrote in their letter. Apple also said it does not yet have a plan in place to protect consumers, other than slowing performance, if it’s unable to solve the battery shutdown issue by the time its reduced-cost battery replacement program ends, the lawmakers wrote.

The letter was also signed by three Energy and Commerce subcommittee chairmen, including Marsha Blackburn (R-Tenn.), Bob Latta (R-Ohio), and Gregg Harper (R-Miss.).

A spokesperson from Apple did not immediately respond to a request for comment.

Senate Commerce, Science and Transportation Chairman John Thune (R-S.D.) Jan. 9 sent a similar letter to Apple requesting more information about the company’s handling of customer complaints and asking if it had considered offering free replacement batteries.