Shin Bet director Yuval Diskin said gunmen loyal to Palestinian President Mahmoud Abbas' Fatah movement planned to attack Olmert's convoy as it entered the West Bank town of Jericho on Aug. 6, according to the participant, who was not authorized to disclose meeting details and thus spoke on condition of anonymity.
Israel notified Palestinian authorities of the plan, and they arrested an unspecified number of Fatah militants, Israeli news reports said. Although the men admitted they planned the attack, the Palestinian Authority released them last week, media reported.
Olmert's office has protested their release to Abbas, they said.
Abbas has little control over pro-Fatah armed factions, which frequently act counter to his efforts to reconcile Israel and the Palestinians.
Israel Radio cited a Palestinian security source as saying three men were arrested, questioned for two months, but were released for lack of evidence.
A government spokeswoman was unable to immediately confirm the reports, which had no further details about how close the militants got to carrying out the plot. It also was not clear why the assassination plan was released 2 1/2 months after it was reported.
The meeting was Olmert's first with Abbas on Palestinian soil after seven years of bloody fighting, and the location was seen as significant as the content.
Israeli and Palestinian security forces worked together to protect the Israeli leader, and Abbas gained some stature by hosting Olmert. Every other meeting has been held at Olmert's Jerusalem residence.