The FBI confirmed Tuesday it was "close" to discovering who is behind the Anthem health insurance hack.
But, the agency's cyber-criminal branch head Robert Anderson said at a briefing with reporters that he would not confirm it until the agency was "absolutely sure," according to one report.
Fingers have already been pointed at China, which has for years been accused of attacking US companies and consumers.
"I don't know if it's China or not," Anderson said, keeping tight-lipped.
As many as 78.8 million people may have been affected by the health insurance hack that hit the company earlier this year. That also includes up to 18.8 million customers of non-Anthem plans, the company said.
It comes as the FBI confirmed Tuesday it has more than 60 different state-sponsored cyber-threat groups on its radar.
Joseph Demarest, the FBI's cyber-crime division assistant-director said the Sony Pictures hack, which crippled computers and networks and destroyed data after a devastating cyberattack blamed on North Korea last year, would not be a "one off."
North Korea strenuously denied the allegations, its state-run media said earlier this year.