WASHINGTON — Congress is spending millions of dollars on investigations of Russia's interference in the 2016 presidential election, but it's impossible to tell from public reports exactly how much the probes are costing taxpayers — and the committees running the investigation refuse to talk about it.

The Senate intelligence committee received an extra $1.2 million earlier this year for its investigation and added two employees to help its existing staff with the Russia probes, according to a congressional aide who was not authorized to speak about the subject publicly. The House intelligence committee also added two new lawyers with a combined annual salary of $290,000, expense reports show.

The congressional investigations are only a portion of the cost to taxpayers of the ongoing Russia saga. Special counsel Robert Mueller is running a criminal investigation out of the Justice Department for which there is little public accounting of the costs, and there is no telling how much the White House legal office is spending to respond to inquiries arising from the investigations.

Congress did not create a special committee for the Russia investigation like it did for the House Select Committee on Benghazi, which spent — and disclosed in routine accounting reports — nearly $7 million from 2014 to 2016 investigating the 2012 killing of the U.S. ambassador and three other Americans at the U.S. compound in Libya.

Spending by special committees is much easier to track because their entire budget supports a single investigation. In contrast, the congressional committees conducting Russia probes are existing panels with sweeping responsibilities and expenses unrelated to Russia.

Monthly expense reports filed by the House intelligence committee show that it has spent more than $3.1 million through August of this year on salaries, travel, equipment and other expenses. That is up nearly $600,000 from the same period a year ago and up more than $800,000 over the same period in 2015. The committee would not provide an estimate of how much of that budget is being consumed by the Russia probe.

Attorneys who have worked on congressional investigations in the past estimate that the House and Senate intelligence committees are spending between 20% and 50% of their time on the Russia investigations.

"I would say that up to 20% of their time is spent on the Russia investigation," said Charles Tiefer, who served as general counsel to the U.S. House of Representatives from 1984 to 1995 and was special deputy chief counsel for the House Iran-Contra Committee's investigation of the Reagan administration. "When you work with the staff of the House intelligence committee, you're struck by the vast scale of their responsibility."

Both the House and Senate intelligence agencies oversee 17 military and civilian intelligence agencies with a total budget estimated at about $75 billion. One of their biggest jobs is to approve an annual bill that authorizes funding for the agencies, a process that Tiefer said involves hours of closed-door, classified briefings from top intelligence officials.

"They're overseeing CIA activities in Iraq and Syria, to give you an example," said Tiefer, who is now a professor at the University of Baltimore School of Law.

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Andrew Wright, former staff director of the House Oversight and Government Reform Committee, said a major investigation typically required more than 50% of his staff's time. But he said it's impossible to say how much time the two intelligence committees are spending on the Russia probe since every committee chairman runs his or her panel differently.

"For an investigation like this, it's got to be massive," said Wright, who is an associate professor at Savannah Law School in Georgia and former associate counsel to ex-president Barack Obama.

The Senate intelligence committee, unlike its House counterpart, is not required to file monthly public expense reports. But, in a resolution submitted to the Senate Rules and Administration Committee in February, intelligence Chairman Richard Burr, R-N.C., estimated that the committee's expenses would be as high as $3.2 million from March through September and as high as $5.5 million from October through September 2018 — the same as the year before — but that was before the committee got the extra $1.2 million for the Russia probe.

The Senate committee also refused to say how much of its total budget is being spent on the Russia investigation.

However, by their leaders' own descriptions, the intelligence committees have been spending a great deal of time on their Russia probes.

At an Oct. 4 news conference, Burr and Vice Chairman Mark Warner, D-Va., said Senate investigators had interviewed more than 100 witnesses for a total of more than 250 hours and created more than 4,000 pages of transcripts. Staff members also had read more than 100,000 pages of documents and the committee had held 11 public hearings on Russia, the senators said.

Burr said the committee planned to interview about 25 more witnesses in October alone.

USA TODAY reported last month that the House intelligence committee paid to send two of its staff investigators to London in July to try to contact Christopher Steele, the former British spy who compiled a dossier on Donald Trump and his alleged ties to the Russian government.

In addition to the two intelligence committees, the Senate Judiciary Committee is conducting a much more limited Russia investigation that focuses mostly on actions by the FBI and the Department of Justice, which it oversees. Spokesman Taylor Foy said the committee had not hired any additional staff to help with its probe.

One of the reasons that congressional committees don't like to disclose how much time and money they are spending on an investigation is that "they don't want to have to admit to the lost opportunity costs" of what's not getting done during the probe, Wright said.

"That's very ingrained in the culture of the Hill," he said. "You don't want to admit that maybe you're letting something else slip."

The committees are being asked to do major investigations with too little staff, Tiefer said.

The House intelligence committee listed 32 staff members on its latest expense report from August. In contrast, the House Select Committee on Benghazi had 46 staffers and the House Iran-Contra Committee had more than 100.

"I think it's deliberately being done on the cheap in order for leadership to limit its scope," Tiefer said.