Yahoo declined to comment for this article, pointing to regulatory filings that say the company is cooperating with government officials - including those from the SEC - examining the hack.
But the investigation's implications stretch far beyond Yahoo's immediate business. It also raises fresh questions for the telecommunications giant Verizon, which is in the midst of a $4.8 billion deal to acquire the former Web titan.
"I think it's going to get a lot uglier for Yahoo going forward over the next year," said Jeff Kagan, an independent technology and telecom analyst.
Verizon declined to comment. But executives have voiced strong concerns about the hacks, signaling in October that they may have had a significant impact on Yahoo's core business. Analysts say that a concrete finding of that sort by Verizon could allow it to renegotiate terms or even abandon the purchase.
Verizon had expected the deal to close in the first quarter. But in its quarterly earnings report Monday, Yahoo said that it now expects the deal to close "as soon as practicable" in the second quarter, citing "work required to meet closing conditions."
In the same report, Yahoo chief executive Marissa Mayer said that attempts to improve the company's security posture were ongoing. About 9 in 10 of the company's daily users have already beefed up their account security by changing their passwords or taking similar steps, or never needed to in the first place, according to the earnings report.
The fate of the deal remains in doubt as Marni Walden, Verizon's president of product innovation and new businesses, said this month that she was unsure whether it would go through.
"I can't sit here today and say with confidence one way or another, because we still don't know," she told an investor conference.
Yahoo shares closed up 0.83 per cent Monday.