"There must be factual evidence beforehand for the attorney-general to be able to decide whether to order police to begin a criminal investigation," Moshe Cohen, a justice ministry spokesman, said.
"To reach such a decision, they asked police to conduct preliminary checks," he said.
There was no immediate comment from Israeli police.
The justice ministry said on Thursday prosecutors were looking into material relating to the sale of Bank Leumi in 2005, when Olmert was finance minister.
Israeli media had said Olmert had allegedly promoted the interests of two overseas businessmen in bidding for the bank, whose controlling interest was sold to US investment group Cerberus-Gabriel for 2.47 billion shekels [$540 million].
Israel's state comptroller, its main government watchdog, told attorney-general Menachem Mazuz last week that it suspected there was prima facie evidence indicating Olmert may have committed a crime, Israeli media reported.
The prime minister, whose popularity has plunged following Israel's war with Lebanon, has already been accused of cronyism in his purchase of a Jerusalem apartment in 2004.
His office has not commented on the issue, but Olmert has said all real-estate deals he had ever conducted were legal.