"I can tell you with 100% certainty that the Russian intelligence services would have been aware of that meeting in advance as they were plotting it out, there would have been weeks spent studying how to best achieve the results in that meeting," Browder told the Senate judiciary committee Thursday.
Browder added that he has no first-hand knowledge of the details of the meeting but was commenting on his experience with the Russian government.
"I have no direct evidence, but I know how Russian security services behave, and this was a big ask, to repeal a major legislation," Browder told the committee. "They wouldn't have gone in without being prepared to offer something in return. What the offer was I don't know. But the KGB and FSB would have studied their targets carefully."
Browder, who worked with the Senate to pass the sanctions almost a decade ago, told members of the judiciary committee Thursday that Russian lawyer Natalia Veselnitskaya is the family lawyer for a Russian family closely aligned with Russian President Vladimir Putin.
Meanwhile, Sen. Lindsey Graham, who is leading one of a handful of Congressional probes into Russia's interference in the election, said he believed the June 2016 meeting with Donald Trump Jr., then-Trump campaign chairman Paul Manafort, Jared Kushner, Veselnitskaya and others was not the only one.
"The idea that Manafort, who is well known by the Russian world as sympathetic to their cause, that they didn't see that as an opportunity to get a relationship with the Trump campaign is hard to believe," Graham said.
Graham then quoted from Trump Jr.'s email exchange where he said he would "love" dirt on the Clinton campaign for use later in 2016.
"It's also hard for me to believe that once the Trump campaign expressed a desire to get help -- 'Yeah, I love it -- maybe later in the summer' -- that that meeting was one and done," he said.
Attendees of the June 2016 meeting have repeatedly said that their was no additional contact between Veselnitskaya and the Trump campaign.
"As it ended, my acquaintance apologized for taking up our time," Trump Jr. said in a statement, obtained by CNN on July 9. "That was the end of it and there was no further contact or follow-up of any kind. My father knew nothing of the meeting or these events."
Trump Jr.'s revelation of the June 2016 meeting with Veselnitskaya has reignited interest in Russia's efforts in the 2016 election for weeks now, spurring congressional investigators to pry into new avenues.
The House and Senate intelligence committees interviewed Kushner earlier in the week -- Kushner, in testimony he released publicly, said that he remembered very little of the meeting and later asked an aide to bail him out of the meeting.
Senate intelligence committee staff interviewed Manafort as well this week. Senate Judiciary Committee Chairman Chuck Grassley subpoenaed Manafort earlier in the week, but dropped the subpoena after Manafort agreed to testify privately and provide his notes about the June 2016 meeting.
The meeting has also sucked in the players in the fight over the Magnitsky Act -- Russian sanctions approved in 2012 -- into the Russian probe. Veselnitskaya, the Russian lawyer in the Trump Jr. meeting, has been the lead proponent in the US of overturning the ban.
The hearing with Browder had originally been planned as a blockbuster event with requests of public testimony from Trump Jr., Manafort and Glenn Simpson, whose firm helped craft the "Steele Dossier" with stunning allegations against Trump. But all three witnesses negotiated with the committee to appear in private instead.