WASHINGTON      -   U.S. House Democrats on Friday subpoenaed the White House for documents they want to see as part of their impeachment investigation of President Donald Trump.

The chairmen of three House of Representatives committees said they want documents related to a July 25 phone call between Trump and Ukrainian President Volodymyr Zelenskiy that is at the heart of their inquiry. The three said they were forced to issue the subpoena after the White House failed to produce documents they requested in a Sept. 9 letter.

“We deeply regret that President Trump has put us - and the nation - in this position, but his actions have left us with no choice but to issue this subpoena,” said Representatives Elijah Cummings of the Oversight Committee, Adam Schiff of the Intelligence Committee and Eliot Engel of the Foreign Affairs Committee.

They gave the White House until Oct. 18 to produce the information, including who else besides Trump was on the phone call with Zelenskiy. “This subpoena changes nothing - just more document requests, wasted time, and taxpayer dollars that will ultimately show the President did nothing wrong,” White House spokeswoman Stephanie Grisham said in a statement.

White House lawyers believe Trump, a Republican, can ignore lawmakers’ demands until the Democratic-controlled House holds a full vote of the chamber to formally approve of the impeachment inquiry, according to a person familiar with the matter. Earlier, the committees asked Vice President Mike Pence to hand over documents relating to a meeting he held with Zelenskiy and the call between Zelenskiy and Trump.

They gave Pence until Oct. 15 to produce any records relating to the July call and a meeting he held with Zelenskiy on Sept. 1. According to a partial transcript of the call, Trump asked Zelenskiy the “favour” of investigating former U.S. Vice President Joe Biden, a leading contender for the Democratic presidential nomination in 2020, and his son, Hunter Biden, who had served on the board of Ukrainian gas company Burisma.

At the time, the Trump administration was withholding hundreds of millions in aid for Ukraine and Democrats said they suspect Trump was using U.S. foreign policy and taxpayer money for his personal political gain. Trump is running for re-election. “Recently, public reports have raised questions about any role you may have played in conveying or reinforcing the president’s stark message to the Ukrainian president,” Cummings, Schiff and Engel wrote in a letter to Pence.

A spokeswoman for Pence said the broad nature of the request showed that it was not “serious.” When Pence met with Zelenskiy, the two discussed the $250 million in security assistance that the U.S. Congress had approved but that the Trump administration had not disbursed.

The contents of the Trump-Zelenskiy call were revealed by a member of the intelligence community who filed a whistleblower complaint and remains anonymous. Trump has said he wants to know the whistleblower’s identity. Democratic Representative Raja Krishnamoorthi, a member of the Intelligence Committee, said he believed the whistleblower’s identity should be kept secret even from members of the panel. “I think that the less we know about that person, the more protected he or she will be,” Krishnamoorthi told Reuters.

A second intelligence official, who has more direct information about Trump’s dealings with Ukraine than the first whistleblower, is also considering filing a whistleblower complaint, the New York Times reported, citing two people briefed on the matter.

The second official is among those interviewed by the intelligence community inspector general to corroborate the allegations of the original whistleblower, one of the people said, according to the Times.