A U.S. House committee on Thursday subpoenaed the country's energy secretary, Rick Perry, as part of the ongoing impeachment inquiry into U.S. President Donald Trump, demanding documents about Trump's dealings with Ukraine.

The subpoena, issued by House Intelligence Committee in consultation with Oversight and Reform and Foreign Affairs committees, requested that Perry produce relevant materials by Oct. 18.

In a letter containing the subpoena that was sent to Perry, House Intelligence Committee Chairman Adam Schiff, Oversight and Reform Committee Chairman Elijah Cummings and Foreign Affairs Committee Chairman Eliot Engel cited media reports as saying that it was the energy secretary who urged Trump to make the phone call to Ukrainian President Volodymyr Zelensky on July 25. The controversial phone call is at the center of the impeachment inquiry.

The chairmen in the letter also raised concerns over Perry leading the U.S. delegation to attend Zelensky's inauguration on May 20. The original arrangement was that the delegation would be headed by Vice President Mike Pence, who reportedly was ordered by Trump not to attend the ceremony.

In addition, Perry was also asked by the chairmen to explain events related to his alleged request to the Ukrainian authorities that they make changes to the advisory board of the country's state-owned oil and gas company, Naftogaz.

Zelensky, for his part, denied that Perry put pressure on him with regard to Naftogaz. "No, there wasn't," he said Thursday in comments to the Kyiv Post.

Furthermore, the chairmen also sought clarification from Perry with respect to allegations that Trump instructed Perry to consult with Rudy Giuliani, the president's personal attorney, in arranging a meeting between Trump and Zelensky.

"The Department of Energy (DOE) is aware of the House Committees' letter to Secretary Perry and it is currently under review by DOE's Office of General Counsel," DOE spokeswoman Shaylyn Hynes was quoted as saying by Politico.

Also on Thursday, the three House panels subpoenaed Lev Parnas and Igor Fruman, Giuliani's two associates who were arrested late Wednesday at Dulles International Airport in Washington when trying to travel to Frankfurt, Germany with one-way tickets.

The two were charged with allegedly violating campaign finance law by funneling foreign money to U.S. politicians. They reportedly helped Giuliani seek evidence of Democratic presidential candidate Joe Biden's alleged involvement in a corruption case in Ukraine.

The Biden investigation, which Trump has said he requested because of corruption rather than political concerns, is part of the reason that brought the president into the impeachment inquiry, and Giuliani under a congressional subpoena.

Biden has adamantly denied any wrongdoing, accusing, instead, Trump of "abuse of power."