Sources reveal that, last year, Yahoo Inc built a program that can search all of its clients' incoming emails. The software has been custom-made for the US intelligence circle which provides the essential elements of information needed to acquire the data.

According to former employees who have knowledge of the events, the search engine giant gives in to the demands of the US government. As ordered by the National Security Agency (NSA) and the FBI, hundreds of millions in Yahoo Mail accounts have been examined and scanned.

Yahoo's participation in the US government's intelligence drive is the first known incident to emerge that involves an American technological company. Instead of checking stored messages or scanning user accounts in real time, the California-based organization screened all incoming emails.

Sources further add that intelligence officials did not inform Yahoo what kind of records they were after. The instruction provided to the tech group only involves a set of characters to be searched. It remains unclear if the US intelligence circle has also approached other tech companies.

Two former employees unveil that Yahoo CEO Marissa Mayer has given the directive to abide with what the US intelligence division requested. The move turns out to be a critical, and to a certain extent, a fortunate one since it prompted Chief Information Security Officer Alex Stamos to leave in June 2015. Stamos now occupies the top security position at Facebook Inc.

In response to Yahoo's breach of user email accounts, the company has reiterated that the search engine giant is a law-abiding organization and it complies with the laws of the United States.

Although the cooperation between intelligence agencies and tech companies is known, surveillance experts and observers find it astounding to learn about the demand for real-time web data including the creation of a program that will harness the gathering of information.

Surveillance operations are usually conducted upon the request of the NSA. Considering that there are a number of agencies out there including the FBI and the CIA, it will be difficult to know which group is actually looking for data.

Meanwhile, Google's Alphabet Inc and Microsoft Corporation have separately stated that no email searches on a grand scale were ever conducted. Yahoo's revelations will likely spark a congressional inquiry about the sweeping powers of US intelligence agencies.