WASHINGTON - The Federal Trade Commission said on Thursday it has opened an investigation into the massive data breach at Equifax Inc, in a rare public disclosure that sent shares tumbling to their lowest in more than two years.

"The FTC typically does not comment on ongoing investigations. However, in light of the intense public interest and the potential impact of this matter, I can confirm that FTC staff is investigating the Equifax data breach," spokesman Peter Kaplan said in a brief email statement.

Equifax shares fell 5 percent to $94.19 in heavy trading after earlier touching $89.59, their lowest since February 2015.

In the first 15 minutes of trading, more than 4.7 million shares had crossed, nearly 10 times the stockRs daily average volume dating to 1980. The activity came on the heels of a record 17.5 million shares traded on Wednesday.

Equifax representatives did not immediately respond to requests for comment on the FTC probe.

Equifax disclosed the breach on Sept, 7, saying thieves may have stolen the personal information of 143 million Americans in one of the largest hackings ever. It learned of the hacking on July 29.

Nearly 40 states have joined a probe of its handling of the breach. Equifax Chief Executive Officer Richard Smith, is expected to testify on Oct. 3 before a U.S. House of Representatives panel.