The Senate Judiciary Committee has subpoenaed the research firm responsible for the bombshell Russia dossier that contains unverified memos related to President Donald Trump's activities in Russia.
An attorney for Fusion GPS cofounder Glenn Simpson rejected a request to voluntarily appear before a Senate Judiciary hearing next week, citing Simpson's vacation plans.
The committee sought to question Simpson amid discussions about the Foreign Agents Registration Act (FARA), which comes in light of accusations that the firm played a role in setting up a June 2016 meeting between Russian lawyer Natalia Veselnitskaya, Donald Trump Jr., and former Trump campaign manager, Paul Manafort.
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Donald Trump and his son, Donald Trump Jr. through the years
Donald Trump and his son, Donald Trump Jr. through the years
FILE PHOTO: Donald Trump Jr. (L) gives a thumbs up beside his father Republican U.S. presidential nominee Donald Trump (R) after Trump's debate against Democratic nominee Hillary Clinton at Hofstra University in Hempstead, New York, U.S. September 26, 2016. REUTERS/Mike Segar/File Photo
Donald Trump, flanked by his son Donald Jr. (R), addresses the media in Chicago May 10, 2006. Trump was in Chicago to speak about his Trump International Hotel and Tower Chicago that is being built on the old site of the Chicago Sun-Times building on the north side of the Chicago River. Trump also stated in the news conference that he has given the project oversight to the children.
FILE PHOTO: Donald Trump Jr. (C) hugs his father, Republican Presidential candidate Donald Trump, as Donald Jr's wife Vanessa (L) walks past after Trump accepted the Republican presidential nomination at the 2016 Republican National Convention in Cleveland, Ohio U.S. July 21, 2016. REUTERS/Mike Segar/File photo
Donald Trump Jr. and Donald Trump during 1988 U.S. Open - September 3, 1988 at Flushing Meadows Corona Park in Queens, New York, United States. (Photo by Ron Galella, Ltd./WireImage)
Donald Trump Jr. (L) walks off stage with his father Republican U.S. presidential nominee Donald Trump after Trump's debate against Democratic nominee Hillary Clinton at Hofstra University in Hempstead, New York, U.S. September 26, 2016. REUTERS/Brian Snyder
Donald Trump and Donald Trump Jr. during 'Veranda: New York's Best at Trump Park Avenue, The Ultimate Showcase Penthouse' Opening Night Reception at The Trump Park Avenue in New York, New York, United States. (Photo by Robin Platzer/FilmMagic)
Donald Trump (C), entrepreneur and host of the television reality series "The Apprentice" poses with his wife and children (L-R) Donald Trump, Jr., Tiffany, Donald Trump, wife Melania, and daughter Ivanka at the party following the live telecast of the finale of season five in Los Angeles June 5, 2006. REUTERS/Fred Prouser
Donald Trump and Donald Trump Jr. (Photo by Ron Galella, Ltd./WireImage)
NEW YORK, NY - OCTOBER 12: Donald Trump (C) and his sons Eric F. Trump (L) and Donald Trump Jr. (R) attend the 'Celebrity Apprentice All Stars' Season 13 Press Conference at Jack Studios on October 12, 2012 in New York City. (Photo by Desiree Navarro/WireImage)
Donald Trump (2nd L), entrepreneur and host of the television reality series 'The Apprentice', poses with his children, (L-R) son Donald Trump, Jr., and daughters Tiffany and Ivanka at the party following the live telecast of the finale of season five in Los Angeles June 5, 2006.
CLEVELAND, OH - JULY 20: Republican presidential candidate Donald Trump (C) speaks with Donald Trump Jr. (L) and Ivanka Trump (R) during the third day of the Republican National Convention on July 20, 2016 at the Quicken Loans Arena in Cleveland, Ohio. Republican presidential candidate Donald Trump received the number of votes needed to secure the party's nomination. An estimated 50,000 people are expected in Cleveland, including hundreds of protesters and members of the media. The four-day Republican National Convention kicked off on July 18. (Photo by Joe Raedle/Getty Images)
Donald Trump (R) waves to a fan as his son Donald Jr. looks on after Trump received a star on the Hollywood Walk of Fame in Los Angeles January 16, 2007. REUTERS/Chris Pizzello (UNITED STATES)
(L-R) Eric Trump, Donald Trump Jr., and Ivanka Trump and Donald Trump attend the ground breaking of the Trump International Hotel at the Old Post Office Building in Washington July 23, 2014. The $200 million transformation of the Old Post Office Building into a Trump hotel is scheduled for completion in 2016. REUTERS/Gary Cameron (UNITED STATES - Tags: BUSINESS POLITICS REAL ESTATE)
U.S. Republican presidential candidate Donald Trump kisses his son Donald, Jr. at a campaign event at Regents University in Virginia Beach, Virginia February 24, 2016. REUTERS/Joshua Roberts
UNITED STATES - APRIL 14: Donald Trump is joined by his son, Donald Jr., at a Park Ave. luncheon hosted by The Donald. (Photo by Andrew Savulich/NY Daily News Archive via Getty Images)
NEW YORK - OCTOBER 27: Donald Trump, Visionary Business Leader award honoree, poses with his children Donald Trump Jr. and Ivanka at Fashion Group International's 22nd Annual 'Night Of Stars' at Cipriani's 42nd Street October 27, 2005 in New York City. (Photo by Evan Agostini/Getty Images)
NEW YORK - SEPTEMBER 19: Donald Trump Jr., Donald Trump, Ivanka Trump and Eric Trump at the Trump Soho Launch on September 19, 2007 in New York City. (Photo by Mark Von Holden/WireImage)
COO Trump International Hotels, Jim Petrus, Ivanka Trump Donald Trump Jr. and Donald Trump at Donald Trump, Ivanka Trump, Donald Trump, Jr.and Eric Trump Launch the N on October 10, 2007 at Jean Georges, Trump International Hotel & Tower in New York City. (Photo by Shawn Ehlers/WireImage)
DATELINE NBC -- Pictured: (l-r) Donald Trump, Natalie Morales, Donald Trump Jr. -- (Photo by: Heidi Gutman/NBC/NBCU Photo Bank via Getty Images)
INDIANAPOLIS, IN - APRIL 27: Republican presidential candidate Donald Trump (L) introduces his son Donald Trump Jr. (R) as he addressing the crowd during a campaign rally at the Indiana Farmers Coliseum on April 27, 2016 in Indianapolis, Indiana. Trump is preparing for the Indiana Primary on May 3rd. (Photo by John Sommers II/Getty Images)
ST LOUIS, MO - OCTOBER 09: Donald Trump, Jr. (R) greets his father Republican presidential nominee Donald Trump during the town hall debate at Washington University on October 9, 2016 in St Louis, Missouri. This is the second of three presidential debates scheduled prior to the November 8th election. (Photo by Rick Wilking-Pool/Getty Images)
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The meeting was held under the pretense that the Trump campaign would receive damaging information on its rival, Hillary Clinton.
Simpson has previously denied any untoward connection with Veselnitskaya, but said Fusion GPS did collaborate on a lawsuit with the Russian lawyer for two years, The Washington Post reported.
Separately, Trump Jr. and former Trump campaign manager Paul Manafort have agreed to negotiate to provide the Senate Intelligence Committee with documents and be interviewed by the panel before a public hearing.
The committee said it will not issue subpoenas to force Trump Jr. and Manafort to appear at the Wednesday meeting, but would reserve the right to do so in the future.
Here's a statement from the committee's ranking Democrat, Dianne Feinstein:
"Glenn Simpson, through his attorney, has declined to voluntarily attend Wednesday's Judiciary Committee hearing regarding compliance with the Foreign Agents Registration Act," Sen. Dianne Feinstein said in a statement on Friday.
"Therefore, a subpoena has been issued to compel his attendance. Simpson's attorney has asserted that his client will invoke his Fifth Amendment rights in response to the subpoena."
Fusion GPS' work on the Trump-Russia dossier, which contains memos compiled by former British intelligence agent Christopher Steele, was unrelated to the June 2016 Veselnitskaya/Trump Jr./Manafort meeting, The Washington Post reported, adding that work on the dossier was conducted on a different timeline.