"That means millions of Americans' data may have been compromised for two years", the senators wrote.
An investigation into those claims, however, led the company to uncover the catastrophic 2014 hack, according to Lord.
Reports indicate that the data breach is "widespread and serious" and could pose major complications for the impending $4.8 billion sale of Yahoo's main business to Verizon.
As investors and investigators weigh the damage of Yahoo's massive breach to the internet icon, information security experts worry that the record-breaking haul of password data could be used to open locks up and down the web.
"It is necessary for consumers to be notified so they can take appropriate steps to protect themselves".
The letter highlights the fact that Yahoo took almost two years to uncover and disclose the breach, during which time "millions of Americans' data may have been compromised".
On Tuesday, six senior United States senators demanded that Yahoo explain why the security breach which resulted in the theft of more than 500 million user account took two years to come to light and called the company's handling of the breach "unacceptable".
In addition to Patrick Leahy, D-VT, the letter was signed by Senators Al Franken, D-MN, Elizabeth Warren, D-MA, Richard Blumenthal, D-CT, Ron Wyden, D-OR, and Edward J. Markey, D-MA. Yahoo declined to comment.
She earlier said at a conference that SEC examiners in recent months have been checking that companies comply with 2011 agency guidance stressing the need to disclose hacks.
On Sept. 22, the tech giant, Yahoo, disclosed that hackers had stolen the encrypted passwords and even personal details of more than 500 million Yahoo accounts in 2014.
InfoArmor concluded the Yahoo hackers were criminal after reviewing a small sample of compromised accounts, Andrew Komarov, the firm's chief intelligence officer, said in an interview.
Yahoo has not yet issued a statement in response to the letter. "The public ought to know what senior executives at Yahoo knew of the breach, and when they knew it".
Yahoo's shares closed up 2.5 percent at $43.37 each in a broadly bullish market on Tuesday.
In an illustration of the confusion about who carried out the hack and why, an NBC News report Wednesday interpreted Komarov's findings as pointing to the Russian government as the ultimate perpetrator.
A USA government source familiar with the Yahoo investigation said there was no hard evidence yet on whether the hack was state-sponsored.