An American financier who made and lost a fortune in Russia is warning that the killing of Russian opposition politician Boris Nemtsov in February was a threatening message to anyone else who opposes the country's president, Vladimir Putin.

Speaking to CTV's Question Period, Bill Browder said he has no doubts that Putin was responsible for the killing of Nemtsov, who was shot while crossing a bridge near the Kremlin on Feb. 28.

"They killed him in cold blood, right in front of the Kremlin, in probably one of the most secure spots on the planet," said Browder.

"Putin has the motive and the means, and nobody else would have done it in the way that they did it. And I think it was a message to me -- and everybody else who's opposing Putin -- that 'we'll come and kill you.'"

Browder says he fears for his life after a major fallout of his own with the Putin government. 

He started the Hermitage Capital Management, once the largest foreign investment fund in Russia, in 1996. But he eventually discovered that all the companies he invested in, including Gazprom, was were "being run by corrupt (Russian) oligarchs who were stealing all the money." 

In an effort to stop the corruption, Browder worked with international media outlets to expose how his money was being stolen.

But when he attempted to fly into Russia in November 2005, he was jailed by uniformed guards at the airport. The Russians had caught onto his investigation. 

"And then the next day, they put me on a … flight back to London and declared me a threat to national security, never to be allowed into Russia again," he said.

Browder hired Russian lawyer Sergei Magnitsky to investigate what the Russian police were doing when they raided his offices. Magnitsky told Browder that while the police did not manage to take any of his money -- he had taken it out of Russia by then -- they stole $230 million in taxes his company had paid to the Russian government. 

Magnitsky was thrown in jail after he exposed the $230-million theft, where Browder said he was tortured in an effort to get him to withdraw his testimony. 

"They put him in cells with no heat and no window panes in December in Moscow. He nearly freezes to death. They put in him cells with no toilet, just a hole in the floor and the sewage would bubble up," said Browder.

Magnitsky became ill and, according to Browder, was eventually beaten to death by eight riot guards while in critical condition in November 2009. 

Following Magnitsky's death, Browder vowed to get justice. He came up with an idea to freeze the assets of, and ban visas for, the individuals who killed Magnitsky. In December 2014, the U.S. government passed the Magnitsky Act.

In retaliation for the sanctions, Browder said he and Magnitsky ordered to stand trial – three years after Magnitsky died. Magnitsky was tried posthumously and Browder in absentia. 

"There was a judge. There was a prosecutor. There was defence attorneys. There was bailiffs. There was guards, and two empty seats in the court room. And they found us both guilty under their trumped up charges," said Browder. 

Browder, who recently released his book Red Notice, chronicling Putin and the corruption in the Kremlin, said the Russians have been after him ever since he began his crusade for justice. 

"They would like to kill me. They've threatened to kill me. They're threatened to kidnap me. They're tried to get me arrested. They're trying to get me extradited. They went to Interpol. They tried to use everything against me.”