Flynn's attorney Robert Kelner did not respond to phone messages and emails seeking comment.
The Senate committee is one of several on Capitol Hill investigating possible collusion between Russia and President Donald Trump's 2016 campaign. Flynn is also the target of other congressional investigations as well as an ongoing FBI counterintelligence probe and a separate federal investigation in Virginia.
Flynn, a retired Army lieutenant general, was fired from his position as Trump's national security adviser in February. At the time, Trump said he fired Flynn because he misled senior administration officials, including the vice president, about his contacts with Russian officials.
The Senate committee issued the subpoena for Flynn's records on May 10 after he declined to cooperate with an April 28 request for documents. That request was similar to ones received by other Trump associates, including former campaign chairman Paul Manafort, Trump associate Roger Stone and former Trump foreign policy adviser Carter Page, a person familiar with the Senate investigation said. That person spoke on the condition of anonymity to discuss confidential details of the committee's investigation.
Page and Stone shared copies of their request letters with The Associated Press. Those letters sought a wide array of electronic and paper records related to any contacts made between people affiliated with the Trump campaign and Russian officials and businesses. Emails, text messages, letters, phone records and information about financial and real estate holdings associated with Russia were among the information requested by the committee.
Nina Ginsberg, a veteran Washington defense attorney with extensive experience in national security cases, said that without an ironclad immunity deal from the committee, Flynn would be exposed to questioning from investigators about any personal documents he gave up.
Providing those records to the committee would provide authorities with "a lot more information and the legal basis for questioning him about them," she said.
Opening himself up to questioning from Senate investigations would be risky for Flynn given his current legal troubles.
Flynn has been under scrutiny from the Justice Department since at least Nov. 30, when the department's Foreign Agent Registration Act unit sent him a letter questioning whether he needed to register as a foreign agent for lobbying work he performed for a Turkish businessman. The letter and initial stories on Flynn's lobbying were first reported by The Daily Caller.
Flynn signed a $600,000 contract last August with a Turkish businessman, Ekim Alptekin. The contract called for Flynn's company, Flynn Intel Group, to gather information on a Turkish cleric living in the United States with the aim of getting a criminal referral against him. Turkish president Recep Tayyip Erdogan has blamed the cleric, Fethullah Gulen, who lives in Pennsylvania, for a failed coup attempt last summer and has pressured the U.S. to extradite him. The U.S. has rebuffed those calls.
Flynn Intel was paid only $530,000 for the work, which ended in November after Trump's election victory. The day of the election, Flynn wrote a pro-Turkish op-ed, which railed against Gulen, in The Hill newspaper. Flynn has denied the op-ed was related to his Turkish work.
The op-ed prompted scrutiny by the Justice Department, and now a full-fledged federal investigation. According to a U.S. official, authorities in the Eastern District of Virginia are examining Flynn's paid foreign consulting work last year for the Turkish-owned company while he was advising Trump's presidential campaign. The official was not authorized to discuss the probe and requested anonymity.
The existence of the investigation was first confirmed by CNN, which reported that federal prosecutors recently sent grand jury subpoenas to Flynn associates and contractors who worked with his firm. Alptekin told the AP in an email that he is not a target of the investigation and has not been contacted by authorities.
Amid Justice interest, Trump formally chose Flynn as his national security adviser. As the AP reported in March , an attorney for Flynn told Trump's presidential transition team— including now-White House counsel Don McGahn — that Flynn might have to register as a foreign agent.
After Trump's inauguration, Flynn's representatives told the White House counsel that Flynn, who was already serving as national security adviser, would be registering with the Justice Department. Flynn finally registered in March, acknowledging his work could have benefited the government of Turkey.