President Trump continues to defend his oldest son for meeting with a Russian lawyer last year, saying most politicians would have done the same.

Donald Trump Jr. says the meeting was primarily about the Magnitsky Act. The U.S. law, passed in 2012, imposes economic sanctions and travel restrictions on Russians named as human rights abusers.

The act is named after Russian tax attorney Sergei Magnitsky, who worked to expose corruption among people linked to the Kremlin. He died in 2009 while being held in a Russian prison. The Russian government says the cause of death was heart failure, but many observers believe Magnitsky was murdered.

Among them is American businessman Bill Browder, who was once the biggest foreign investor in Russia. He has since become a vocal critic of the country and has clashed with President Vladimir Putin's government.

Magnitsky was Browder's tax attorney, and Browder was a driving force behind the legislation. Browder believes it's Putin's No. 1 priority to get the U.S. to lift sanctions under the act, which currently affect 44 Russians.

Businessman Bill Browder says the Russian lawyer who met Trump Jr. met with was "probably the most aggressive" person he's ever encountered. CBS News

During a conversation on "CBS This Morning," Browder said Russian lawyer Natalia Veselnitskaya, who met with Trump Jr. in June 2016, "was taking money from a Russian oligarch, who is close to Putin, to try to overturn the Magnitsky Act." Veselnitskaya hired Rinat Akhmetshin — whom Browder describes as a "shady former Soviet spy, current spy, Washington operator" — and organized a full-on lobbying campaign, "hiring the top lobbyists, the top law firms, the top PR firms," to try and get rid of the act.

The Trump family claims nothing came out of the meeting. White House chief of staff Reince Priebus called it a "big nothing burger."

"Let's just look at it very simply," Browder said. "Vladimir Putin wants to get rid of this act that's going to sanction his assets. It's his top priority. He assigns an oligarch to go in and spend all the money to get rid of it. The Russian KBG is not stupid. They want something in return."

"We don't know what happened in that meeting," he continued. "We don't know who said what to whom because you can't trust the Russians, and the Trump people keep changing their story, so, who know's what kind of burger it is."

"CBS This Morning" co-host Charlie Rose called attention to Browder's description of Veselnitskaya as "probably the most aggressive person I have ever encountered in all of my contacts with Russians" — to which Browder replied, "Yes, she's a remarkable person. I should caveat that: she's not aggressive in a physical way."

"They were spending money on every different legal motion they could come up with," he continued. "They were hiring lobbyists left and right and center."

"They were getting Donald Trump Jr., all on behalf of Vladimir Putin to get rid of the Magnitsky Act," Browder said.

When asked if he believed there was collusion between the Trump campaign and Russia to impact the American election, Browder replied: "I have no idea. All I can say is I know the Russian side intimately and I can tell you this was a highly resourced operation to get rid of a piece of legislation that would affect Vladimir Putin personally."

The dispute over Magnitsky Act sanctions wound up entangling some American families who were seeking to adopt children from Russian orphanages.

"Vladimir Putin was so angry about the Magnitsky Act that he was looking for some type of retaliation," Browder explained. "He couldn't freeze assets or other types of things because the Americans would retaliate against that. And so he came up with the most heartless, vindictive thing he could do, which was Americans were adopting disabled Russian orphans, and he said, 'No, you can't adopt them anymore.'"

According to Browder, at that time about 500 American families had met sick babies and children who were "longing to go home to America." Some ended up dying in orphanages because they weren't being treated properly.

Browder also addressed the fact that he's been "threatened on a number of occasions by agents of the Russian government."

"I do fear for my life," he said.