The Senate intelligence committee has subpoenaed Donald Trump's former national security adviser Michael Flynn for documents related to the panel's investigation into Russia's election meddling.
Richard Burr, the Republican chairman, and Mark Warner, the Democratic vice chairman, issued a joint statement saying the panel decided to issue the subpoena after Mr Flynn, through his lawyer, declined to co-operate with an April 28 request to turn over the documents.
Mr Flynn and other associates of President Trump have received similar requests from the committee for information and documents over the past few weeks.
Copies of request letters sent to long-time Trump associate Roger Stone and former foreign policy adviser Carter Page were shared with the Associated Press.
Those letters, which were nearly identical, sought emails, text messages, letters, phone records or any other relevant information they have about meetings or contacts that they or any other individual affiliated with the Trump campaign had with Russian officials or representatives of Russian business interests.
They also ask for information about any financial or real estate holdings related to Russia, including any since divested or sold.
Mr Stone, Mr Page, Mr Flynn and former Trump campaign chairman Paul Manafort all received similar requests for information, a source familiar with the Senate investigation said.
The requests sent to Mr Stone and Mr Page covered documents and information from June 2015 to January 20 this year.
During that period, Mr Flynn accepted tens of thousands of dollars from a Russian state-sponsored television network. He later worked as a foreign agent on behalf of a Turkish businessman, while serving as a Trump campaign adviser.
It also covers the post-election time period in which Mr Trump and his transition team decided to appoint Mr Flynn as national security adviser.
Mr Flynn was fired by the president after less than one month in the job. The White House said he misled vice president Mike Pence and other senior officials about his communications during the presidential transition with Moscow's ambassador to the US.
The subpoena comes as the Senate committee and its counterpart in the House of Representatives investigate Russia's interference in the 2016 election and whether Trump associates colluded with those attempts to sway the election.
Mr Flynn's Russia ties are also being scrutinised by the FBI as it conducts a similar investigation.