LAGUNA BEACH, Calif.—The top U.S. prosecutor investigating tech giants like Facebook Inc. and Google made clear Tuesday that no measure is out of his reach.

Makan Delrahim, head of the Justice Department’s growing antitrust division, raised the spectre of breaking up big Silicon Valley names, calling such an action “perfectly on the table.” That is a chilling prospect for the largest companies, which haven’t faced such a threat since the government sought a breakup of Microsoft Corp. two decades ago.

Speaking...