Thomas Lenard, TPI's president, questioned how FTC investigators could tell if Google's search results were biased. "If you were at the FTC, what would you be looking at?" he said.

Determining search bias will not be easy, Bracha said. "One cannot imagine equal, neutral treatment for everybody in this context," he said. "Search, in order to be useful at all, is inherently hierarchical. Somebody has to be at the bottom."

But the American Homeowners Grassroots Alliance, an advocacy group that doesn't disclose its supporters, has heard "countless complaints" from small-business marketers of niche products who see their Google search rankings drop for no apparent reason, said Bruce Hahn, the group's president.

"I don't know what other explanation there could be, other than there's some bias in Google search, and for what reasons, other than financial gain?" Hahn said.

Friday's Google antitrust forum may be a preview of a . The Senate Judiciary Committee's antitrust subcommittee will host a hearing entitled, "The Power of Google: Serving Consumers or Threatening Competition?" at 2 p.m. that day. Eric Schmidt, Google's executive chairman, is scheduled to be a witness.