"Should the suspicions prove to be true or should the police come up with solid evidence, I doubt whether the prime minister will be able to remain in office," the source said.
Another source involved in the investigation told Ynet, "This is a serious affair and a dramatic development."
A comprehensive gag order has been issued encompassing all details pertaining to the case. Sources in the Justice Ministry have estimated that the gag order would be examined once again next week, and that several details would be cleared for publication. At the moment, however, no additional details have been released due to the sensitivity of the investigation.
The prime minister was questioned under caution Friday morning for about an hour and a half. His former bureau chief, Shula Zaken, was also questioned on the same affair.
Head of the National Fraud Unit, Brigadier-General Shlomi Ayalon, arrived with two other investigators at Olmert's Jerusalem residence Friday morning at around 10 am. Once the interrogation ended, about 90 minutes later, the three police officers reportedly rushed to Attorney General Menachem Mazuz's office to brief him on the developments in the case and decide on future proceedings.
Olmert, said the Prime Minister's Office in a short statement issued Friday afternoon, cooperated with his investigators in full, and will continue to do so.
The urgent probe, proxy of a special order issued Thursday night by Mazuz, was the opening act in another investigation launched against Olmert.
The new investigation is not directly linked to one of the ongoing investigations against Olmert, but its leads did surface while the police were working on one of them.
Shula Zaken, who served as Olmert's bureau chief for years, was reportedly interrogated by the National Fraud Investigations Unit on Tuesday.
Zaken has been questioned by the police in the past in connection to other investigations held against the prime minister, as well as in connection with an affair which exposed alleged corruption in the Israel Tax Authority.
At this time, Olmert is facing three different investigations related to the time he served as the minister of trade, labor, and commerce.
Efrat Weiss contributed to this report