Washington - A long-time confidant of Hillary Clinton has been subpoenaed to testify before a special House panel investigating the deaths of four Americans at the US diplomatic post in Benghazi, Libya, according to an official familiar with the probe.This official, speaking to The Associated Press on condition of anonymity, said the subpoena was issued by the panel headed by Representative Trey Gowdy. The official declined to be identified publicly because he wasn't authorised to discuss by name an investigation still in progress.The panel has "a number of questions" for Sydney Blumenthal, the official said, including whether he had any business arrangements that led him to send emails to Clinton and other officials concerning Libya while Clinton was secretary of state. Blumenthal was not an employee of the State Department at the time.Clinton had initially been expected to testify this week on the September 2012 attacks that killed four Americans, including US ambassador Christopher Stevens, but her testimony was put off after Gowdy complained that he lacked the necessary State Department documents to thoroughly question her.Clinton, who is seeking the Democratic Party's presidential nomination, was in Iowa on Tuesday for a campaign appearance.Representative Elijah Cummings, ranking Democrat on the panel, issued a statement on Wednesday assailing the committee's GOP leadership for its handling of the subpoena."There was no need for the Select Committee to send two US Marshals to the home of Sidney Blumenthal... especially since the committee never bothered to contact him first to ask him whether he would voluntarily come in," Cummings said.Gowdy complained to Kerry last week that the State Department had delayed providing emails and other documents involving Clinton and her top associates.The State Department said the department has provided 300 emails from Clinton, 19 witnesses and more than 40 000 pages of documents from the initial departmental investigation.Democrats have complained that Gowdy and other Republicans are drawing out the Benghazi hearings in order to disrupt Clinton's presidential campaign, but Gowdy said that the State Department is to blame for any delays.Jamal Ware, a spokesperson for the Benghazi committee, said on Tuesday that "questions about why it will take so long and how much it will cost the department to comply with public records requests due to challenges stemming from former Secretary Clinton's personal email arrangement are best directed to State and the former secretary".The New York Times reported earlier this week that Blumenthal, 66, had written then-Secretary Clinton a host of emails about Libya before and after the death of Muammar Gaddafi. Blumenthal was a White House aide during Bill Clinton's presidency.Clinton said on Tuesday she wants the State Department to speed release of her emails.