At least 15 advertising-technology companies had sufficient information to detect that publisher Gannett Co. provided inaccurate data to advertisers for more than nine months, but the firms failed to connect the dots and alert their clients, according to researchers who studied the incident.

Gannett, owner of USA Today and news outlets in 46 U.S. states, said it unintentionally provided the wrong information in online ad auctions, due to an error that affected its systems from late May 2021 to early March of this year.