WASHINGTON (AP) — Impeachment investigators issued subpoenas Friday to three more Trump administration officials, demanding their testimony in the probe of President Donald Trump's efforts to force Ukraine to feed him damaging information about his Democratic political opponents.

The chairs of the House committees leading the impeachment inquiry subpoenaed two officials of the White House Office of Management and Budget: acting director Russell Vought and Michael Duffey, who oversees national security programs.

They also subpoenaed State Department counselor T. Ulrich Brechbuhl.

Investigators asked all three earlier this month to testify, but none have appeared.

As House investigators pushed forward, Trump continued his defiance with another round of combative verbal volleys.

He predicted that if the House impeaches him and the Republican-run Senate holds a trial on whether to remove him from office, he would prevail "for one reason: I did nothing wrong."

Speaking to reporters as he left the White House for an appearance in South Carolina, he said people are "angry" because "this isn't a takedown of the president, this is a takedown of the Republican Party."

He also renewed his assertion that the impeachment effort is endangering the economy. He said that "if anything ever happened," the result would be "a recession, depression the likes of which this country hasn't seen."

The Trump administration has refused to make its officials available for depositions in the investigation and resisted supplying documents as well. But witnesses have been appearing anyway after they are issued subpoenas, often on a daily basis for hourslong appearances behind closed doors.

"The committees therefore have no choice but to issue a subpoena compelling your mandatory appearance," the letters read.

Investigators want to know why nearly $400 million in military aid to Ukraine was delayed, even though it was approved by Congress and signed into law by Trump.

Ukraine has relied on U.S. help during a five-year war with Russian-backed separatists in the country's east, where the rebels control territory. More than 13,000 people have died in the fighting.

Brechbuhl is said to have been the source of a mysterious packet of materials that House investigators were given. Rep. Jamie Raskin, D-Md., has said the package contained information from debunked conspiracy theories about Ukraine's role in the 2016 election. The significance of the packet is unclear.

Others have testified that Trump was demanding investigations of Democrats in exchange for the aid and for an Oval Office meeting coveted by Ukraine's new president, Volodymyr Zelenskiy.

The committees want Duffey to appear on Nov. 5, and Vought and Brechbuhl the following day.