Washington

The seemingly dubious "dossier" on Donald Trump's pre-election involvement with Russia got a major credibility boost when the former investigative reporter who commissioned it appeared before Congressional investigators and provided reams of backup material, according to a source with knowledge of his testimony.

Glenn Simpson, who once worked for The Wall Street Journal, testified Tuesday behind closed doors before staff members of the Senate Judiciary Committee. He provided somewhere in the range of 40,000 pages of documents to back up the dossier's allegations that Trump had at least a five-year history of ties to Russia, the source confirmed.

The testimony and documents "give the dossier some greater factual support and context, and possibly also more enhanced public prominence," said Sen. Richard Blumenthal, D-Conn., a Judiciary Committee member. "At this point, the significance of Glenn Simpson in the overall investigation (of Trump's Russia connections) is hard to assess without more facts."

Blumenthal was briefed on Simpson's interview, but declined to discuss it.

Simpson's lawyer, Josh Levy, told reporters that "Mr. Simpson told Congress the truth and cleared the record on many matters of interest to congressional investigators."

Levy said Simpson's firm, Global GPS, is "proud" of the dossier and "stands by it."

BuzzFeed put the dossier online days before Trump's inauguration in January. Trump dismissed it as "fake news" and "made-up junk." At first, it failed to gain traction because its 35 pages of eye-popping allegations were thought to be unverifiable.

Simpson assigned the writing of the dossier to a former British intelligence officer, Christopher Steele, who had once been stationed in Moscow. In his testimony, Simpson declined to name those who in turn had hired him. Press reports have speculated two clients were behind it, one Democrat and one Republican.

Among other things, the dossier stated that Russian authorities had been cultivating and supporting Trump's political ambitions for at least five years.

It also alleged that the Russian state security service arranged for a Trump encounter with prostitutes in a hotel room once occupied by the Obamas on a visit to Moscow, and had secretly videotaped their cavorting for possible blackmail purposes. Trump has vigorously denied it.

Special Counsel Robert Mueller is conducting a wide-ranging investigation into Russian involvement in the 2016 election.

Two weeks ago, FBI agents under Mueller's direction executed a search warrant on the Alexandria, Va., home of former Trump campaign manager Paul Manafort.

The Manafort raid and the empaneling of a grand jury are milestones in the ongoing Mueller investigation, which is said to encompass allegations that Trump obstructed justice by firing former FBI Director Comey after Comey declined to drop the probe.

Simpson is just one of several witnesses to appear before the committee, Blumenthal said, adding that Manafort and Donald Trump Jr. are likely to be more significant in terms of filling in blanks. Both men participated in the key meeting at Trump Tower in June 2016 in which a "Russian government lawyer" was to disclose damaging information about Clinton that Trump could use to his advantage.

Trump Jr. and Manafort were subpoenaed last month to appear before the committee, but Judiciary chairman Sen. Charles Grassley, R-Iowa, took them back so that staff members could continue to negotiate their appearances. Those negotiations are continuing.

dan@hearstdc.com