"Heightening concerns, moreover, were reports that Apple had been informed of Adware Doctor’s actions for at least several weeks, but did not pull the Adware Doctor app until these actions were made public," Rubio said.
The senator from Florida characterized Apple’s response as "extremely disconcerting" and underscored he found it alarming that the company could not detect the application's covert collection and storing process.
Rubio has been a vocal critic of China's trade policies and praised the US President Donald Trump's strong stand on the issue, including Trump introducing tariffs on Chinese imports.
The threat of US user data being stored on China's servers is also very troubling, Rubio said in the letter.
Rubio asked Cook to answer several questions pertaining to the incident, including why the reports about Adware Doctor were not immediately investigated and what steps would Apple undertake to prevent similar incidents in the future.