A report in The Wall Street Journal said the letter indicated that privacy concerns, that surfaced when Facebook was found to have provided user information to data-analytics firm Cambridge Analytica, had now spread to other companies.
Members of the Energy and Commerce Committee in the US House of Representatives said in the letter that their concerns had arisen following recent media reports.
One report was about Google allowing third-party developers to have access to users' Gmail accounts.
The politicians said "this alleged behaviour is troubling", in the letter, which was signed by committee chairman Republican Greg Walden of Oregon, and three sub-committee chairmen: Republican Representatives Gregg Harper of Mississippi, Marsha Blackburn of Tennessee and Robert Latta of Ohio.
The WSJ said the letter sent to Cook raised fewer issues but asked about the collection and transmission of location data. It said that statements made by Cook and Apple's actions “raise questions about how Apple device users’ data is protected and when it is shared and compiled".
Facebook is already under investigation by government bodies, including the Federal Trade Commission.
The letters are the first to ask any company apart from Facebook about privacy practices.