Delrahim said at a tech conference in Colorado the government is looking at previously approved acquisitions as part of a broad antitrust review announced in July of major tech firms with significant market power.
"Those are some of the questions that are being raised ... whether those were nascent competitors that may or may not have been wise to approve," Delrahim said. "Whether the intention of the incumbent was to purchase some of those competitors, I don't know. I'm not privy to the facts of each of those investigations."
On July 23, the Justice Department said it was opening a broad investigation into whether major digital technology firms engaged in anticompetitive practices, including concerns raised about "search, social media, and some retail services online."
That was an apparent reference to Alphabet Inc's Google, Amazon.com Inc and Facebook Inc, and potentially Apple Inc.
More than a dozen states are expected to announce in the coming weeks they are launching a formal probe, a person briefed on the matter told Reuters.
"I think it's safe to say more than a dozen or so state attorneys general that have expressed an interest in the subject matter," Delrahim said.
US Representative David Cicilline, a Democrat who chairs a House panel that oversees antitrust issues, said on Tuesday, "Big tech platforms aren't going to regulate themselves. I'm pleased that several state attorneys general are now conducting their own investigation into the dominance of these firms."
In July, eight state attorneys general met with US Attorney General William Barr to discuss the effect of big tech companies on competition, and various antitrust actions.
The New York Attorney General's office said on Monday it is continuing to "engage in bipartisan conversations about the unchecked power of large tech companies." North Carolina's attorney general, Josh Stein, is also "participating in bipartisan conversations about this issue," his office said.
The department is looking not only at price effects, but also innovation and quality, and the next steps in its broad antitrust review would be seeking documents and other information and could include some compulsory requests, Delrahim said.
After the July announcement the companies under investigation "immediately reached out to work with us in a cooperative manner to provide information that we need as far as the investigation," Delrahim added.
The Federal Trade Commission in June told Facebook it had opened an antitrust investigation. Last month, the FTC resolved a separate privacy probe into Facebook's practices after the company agreed to pay a $5 billion penalty.