The five Democratic senators want Horowitz to "conduct a thorough investigation" into Session's decision.

"We ask that you consider whether and when the Attorney General consulted with ethics officials or others regarding his involvement in these investigations, his contacts with Russian officials, and his testimony before our Committee during the confirmation process," the senators wrote.

The senators also want him to examine any communications between Sessions and the White House about the recusal or any investigation, as well as if, and to what extent, Sessions was previously involved in the investigations.

"Please recommend appropriate action to address any problems that you discover," they added.

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The Washington Post reported on Wednesday night that Sessions spoke to Russian Ambassador Sergey Kislyak during the presidential campaign but denied any such meetings during his confirmation hearing when he was under oath.

The five Democrats argued that his statements to the Judiciary Committee were "at best ... incomplete and misleading."

Sessions announced on Thursday that he would recuse himself from any current or future investigations, but stressed that his decision wasn't an acknowledgment of wrongdoing. He also said that his staff had been reviewing, prior to the Washington Post story, if he should step back from any investigation involving the Trump campaign.

Democrats, who have a minority in both Houses, have limited leverage within Congress either bring Sessions back to testify or force an independent commission.