Prime Minister Ehud Olmert announcement on Wednesday that he would not run for reelection in the Kadima primary on September 17 was because "he recognized that the investigation evidence held by police is serious and solid," law enforcement sources said on Wednesday night.

Olmert will be interviewed by police on Friday at his residence in Jerusalem - the fourth such meeting with investigators in recent months. The interview is expected to focus mostly on the "Olmertours" affair, according to police sources.

The case in question involves payments made to Rishon Tours, a travel agency that catered to the needs of Olmert and his family.

"The investigation Friday is expected to be difficult and uncomfortable for Olmert," the law enforcement sources said. "He will be confronted with evidence and documents that have accumulated against him, and it is a fair assumption that he already understands that this involves substantive evidence," they added.

During the two-hour interview - which is the time alloted to the police by Olmert's office - the investigators are expected to ask Olmert to explain a series of documents, collected by police during the past month, and which allegedly bolster the suspicions that the prime minister was aware and was party to the use of a mechanism for which he received multiple-funding for his trips abroad. Police suspect the excess funds were used to pay for dozens of flights for Olmert's family over the years.

Police are focusing their investigation on the years when Olmert served as minister of Industry and Trade from 2003 to 2006.

Law enforcement sources suggested in recent days that the investigators may surprise Olmert with questions about other investigations being conducted against him.

But the focus will be Olmertours, and the prime minister will be asked to provide explanations on a precise list of flights for which he allegedly sought multiple funding, and the names of the institutions that funded the trips.

In addition, a list of dozens of flights taken by his family over the years and were allegedly paid for by such external funding.

For the first time, the prime minister will be asked to comment on documents that the investigators collected as part of their probe into the affair.

In addition, the police intend to ask Olmert about internal communications where his signature appears, and which allegedly show that he authorized the various travel plans.

Law enforcement sources stressed in recent days that the evidence in the investigation is solid, and voiced hope that in less than two weeks this probe will be completed. By comparison, the collection of evidence in the Cash Envelopes Affair involving the American businessman Morris Talansky has not yet been completed, even though the case had been under investigation for significantly longer.

The same sources asserted that there is "a high probability" that Olmert will be indicted in the Olmertours affair.

As far as the police investigators are concerned, in spite the prime minister's announcement last night that he will not seek reelection, meaning that he will relinquish the prime minister's office in a few months, they intend to carry on with their various probes.

"Olmert did not resign," a law enforcement source said on Wednesday. "From our point of view, he is still a serving prime minister and he has immunity. This has many implications on the investigation."