Hawley, a Republican, is also questioning Google's alleged practice of "scraping" of online content from competing websites.
Hawley's office argues that federal regulators were wrong not to sue Google and that inaction left an opening for a potential state suit.
Claire McCaskill's seat in 2018, told reporters that he issued an "investigative subpoena" to the tech giant to gather information.
The Kansas City Star first reported on the investigation.
In a statement, Google spokesman Patrick Lenihan said that his company has not yet received Hawley's subpoena.
Federal regulators in the US also have investigated the company over antitrust claims, but Google settled with the Federal Trade Commission in 2013 without making any major concessions on how the company runs its internet search engine. He says Google "collects substantial information" about consumers' use of services like the shopping search engine "Googe Shopping", and airline booking service "Google Flights".More news: Chevrolet Corvette ZR1 is GM's most powerful vehicle ever
In addition to online users' location, device information, cookie data, online queries, and website history, Hawley says it is estimated that Google has access to 70% of all card transactions in the United States. The attorneys general of Utah and the District raised a flag past year, urging the Federal Trade Commission to reopen its investigation into Google's search practices, although the agency has not said it would do so.
The investigation comes as once overwhelmingly positive public opinion about tech companies has started to shift.
Hawley noted that the FTC had chosen not to take enforcement action against Google after a 2012 investigation of its search practices.
Google was not immediately available to comment.
"This is not a "Dear Google" letter", he said.