Yahoo is still reeling from its massive data breach exposé after the public learned last week that hackers had allegedly stolen the login details of at least 500 million accounts in 2014.

But apart from the hackers breaking in, Yahoo may also be responsible for a certain kind of email hacking of its own doing, according to reports.

Reuters published a report on Tuesday that Yahoo had built a custom-made system that not only stored messages but also monitored, in real time, the email accounts of Yahoo users, at the request of the National Security Agency. Sources cited by Reuters, however, did not identify the type of data handed over to the NSA.

The incident, considered to be the very first of its kind, was supposedly the reason behind the resignation of then-security chief Alex Stamos, who is now heading up security at Facebook.

NSA whistleblower Edward Snowden himself weighed in on the matter.

"Any major email service not clearly, categorically denying this tomorrow — without careful phrasing — is as guilty as Yahoo," tweeted Snowden, who in 2013 exposed details regarding the government's surveillance tactics.

Tech companies were quick to deny similar acts.

"We have never engaged in the secret scanning of email traffic like what has been reported today about Yahoo," a Microsoft spokesperson wrote to Vocativ about the issue. Microsoft has been firm in its stance but does not deny having been approached.

A Google representative took a firmer stand on the issue.

"We've never received such a request, but if we did, our response would be simple: 'No way'," shared the company representative.

Facebook, on the other hand, has "never received a request like the one described in these news reports from any government."

"And if we did, we would fight it," a spokesperson from Facebook told Vocativ.

Neither has Twitter received such a request.

"Were we to receive it we'd challenge it in a court," said one Twitter representative when discussing the matter with BuzzFeed.

As for Cupertino, Apple is famous for its strident stance on privacy but says it has never received any requests from the NSA to perform surveillance on users.

"We have never received a request of this type. If we were to receive one, we would oppose it in court," said an Apple representative when asked about Yahoo's current predicament.

Yahoo has become subject to scrutiny lately, especially after Reuters published its report this week. Some have been quick to judge Yahoo's actions as unconstitutional and unprecedented while others have questioned whether or not the NSA has violated, or at least changed, its rules and failed to inform the public.

So far, Yahoo has not specifically tackled the accusations but says it is complying with U.S. laws on such matters.