WASHINGTONWASHINGTON (Reuters) - The Senate's main investigation into allegations of Russian meddling in the 2016 U.S. presidential election is equipped with a much smaller staff than previous high-profile intelligence and scandal probes in Congress, which could potentially affect its progress, according to sources and a Reuters review of public records.

With only seven staff members initially assigned to the Senate Intelligence Committee's three-month-old investigation, progress has been sluggish and minimal, said two sources with direct knowledge of the matter, who requested anonymity.

A committee aide, also speaking on condition of anonymity, said two more staff members were being added and a few others were involved less formally.

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"We need to pick up the pace," Senator Martin Heinrich, a committee Democrat, told Reuters on Monday. "It is incumbent on us to have the resources to do this right and expeditiously, and I think we need additional staff."

While some directly involved in the investigation disputed characterizations of the probe as off track, the appearance of a weak Senate investigation could renew calls by some Democrats and other Trump critics for a commission independent of the Republican-led Congress to investigate the allegations.

The intelligence committees of the Senate and House of Representatives have taken the lead in Congress in examining whether Russia tried to influence the election in Republican Trump's favor, mostly by hacking Democratic operatives' emails and releasing embarrassing information, or possibly by colluding with Trump associates. Russia has denied such meddling.

With the House intelligence panel's investigation for weeks stymied by partisan squabbles, the Senate committee's parallel probe had appeared to be the more serious of the two, with Republican Chairman Richard Burr and top Democrat Mark Warner promising a thorough and bipartisan effort.

Burr, a member of Congress since 1995, last month called the Russia probe one of the biggest investigations undertaken in Congress during his tenure.

Previous investigations of national security matters have been much larger in terms of staffing than the one Burr is overseeing, according to a review of official reports produced by those inquiries, which traditionally name every staff member involved.

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A House committee formed to investigate the 2012 attacks on a U.S. diplomatic mission in Benghazi, Libya, that killed four Americans had 46 staffers and eight interns.

The Senate Intelligence Committee's years-long study of the CIA's "enhanced" interrogation techniques during President George W. Bush's administration had 20 staff members, according to the panel's official report.

A special commission separate from Congress that reviewed the intelligence that wrongly concluded former Iraqi President Saddam Hussein possessed weapons of mass destruction ahead of the 2003 invasion of Iraq involved 88 staffers.

A special Senate committee's 1970s investigation into Watergate-era surveillance practices tapped 133 staffers.

A joint House-Senate probe of the 1980s Iran-Contra affair during Ronald Reagan's presidency involving secret sales of arms to Iran to try to win the release of American hostages, with proceeds going to Nicaraguan rebels, had 181 staffers.

Spokeswomen for Burr and for Warner declined to comment on the staffing levels.

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'INDICATORS OF A COMMITMENT'

The listed sizes of various investigations may be an imperfect comparison because not all staffers listed may have actually had a substantial role, congressional sources said. Investigations often grow in size over time, and a committee aide said the panel had secured $1.2 million in additional funding for the Russia election investigation.

But the numbers are still broadly "relevant as indicators of a commitment to an investigation," said Steven Aftergood, a secrecy expert with the Federation of American Scientists.

"For this investigation to be successful, the committee must recognize the enormity of the job and provide the resources to tackle it," Senator Ron Wyden, another committee Democrat, said in a statement.

Wyden sent a letter last month to Burr and Warner requesting that the probe include a thorough review of any financial ties between Russia and Trump and his associates.

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None of the staffers possess substantial investigative experience or a background in Russian affairs, two of the sources said.

The investigation has not yet conducted interviews with Trump associates suspected of having links to Russian intelligence services, two sources and the aide said.

The investigators have focused on reviewing thousands of pages of documents supporting a previous U.S. intelligence agency finding that Russia interfered to help Trump, and have spoken with intelligence officials in preparation for interviewing key witnesses, they said.

The House intelligence panel's chairman, Republican Devin Nunes, who was a member of Trump's presidential transition team, on April 6 stepped aside from leading that probe because he was under investigation by the House ethics committee for allegedly disclosing classified information.

The Federal Bureau of Investigation is also conducting a wide ranging counter-intelligence investigation into alleged Russian interference and potential collusion with Trump associates, though its findings may never become public.

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