EU antitrust regulators sharpened their scrutiny of how Google ranks its web search results, asking rivals whether their lower rankings affected the number of visitors to their sites, a European Commission questionnaire showed.
The two-page document seen by Reuters came after the EU competition authority demanded more concessions from the world's most popular search engine earlier this month to allay concerns that it blocks competitors in search results.
European Competition Commissioner Joaquin Almunia said Google's offer earlier this year to label its own products in search results, provide links to at least three rival sites and make it easier for advertisers to move to rival platforms was not enough.
His comments echoed those of Google's complainants, such as British price comparison site Foundem, a group of German publishers and online travel site Expedia, which said the concessions could reinforce Google's dominance.
The list of six questions focused on the last two and a half years, as EU regulators sought evidence of any possible link between the complainants' lower rankings in Google search results and lower traffic to their sites.
"In the period from January 2011 to June 2013, have you ever noticed a decrease in the number of users reaching your vertical search sites via Google's natural search, which cannot be explained by a change in your website?" the questionnaire asked.
"Did it coincide with a significant change in the ranking of the pages of your website in Google's natural search results?"
Respondents were given until Aug. 16 to reply to the questionnaire.
Google, which has more than 80 percent of the European search market, reiterated comments made two weeks ago in response to the commission's ultimatum.
"Our proposal to the European Commission clearly addresses their four areas of concern. We continue to work with the commission to settle this case," said spokesman Al Verney.
The company could face a fine as much as $5 billion if it does not resolve the three-year long EU investigation.