Amazon and Facebook declined to comment.
An Apple spokesman pointed to the company’s previous statements, which have said that Apple faces fierce competition and helps thousands of companies reach customers. A representative for Google pointed to testimony from Adam Cohen, its director of economic policy, to Congress in July.
“In the face of intense competition, we are proud of our record of continued innovation,” Mr. Cohen said at the time. “We have helped reduce prices and expand choice for consumers and merchants in the U.S. and around the world.”
The state attorneys general offices did not make any formal comments on plans for an investigation. The tentative timing is that an announcement will be made in early September.
But the states did form a bipartisan, multistate unit called the Tech Industry Working Group months ago.
“As attorneys general, we need to evaluate and address specific conduct, utilizing our existing antitrust and consumer protection laws,” Jim Hood, the attorney general of Mississippi, a Democrat, said in a statement. He added that the working group was “looking at the intersection of technology and antitrust.”
What action, if any, should be taken to curb the market power of the big tech companies is uncertain. Proposals vary from breaking them up, a step endorsed by Senator Elizabeth Warren in her presidential campaign, to forcing them to share data to make it easier for rivals to compete.
But there is a growing criticism of the big tech companies, and increasing calls to rein them in.
In a congressional hearing last month, Representative David Cicilline, Democrat of Rhode Island, and chairman of the House antitrust subcommittee, said that the tech companies had brought innovation and growth to the economy.