An American-born traveller being allowed to enter America isn't usually news — unless that American is a bitter foe of Vladmir Putin, accused of crimes by the Russian leader, placed on an international police watch list and barred from his native country.

Bill Browder is that natural-born American.

Born in the U.S., he found himself temporarily blocked by the U.S., in a bizarre ordeal that revealed the sensitivity of his cause against the Russian government, which hit a new peak following the adoption of a Canadian law that piqued Putin.

In a case with international repercussions, Browder was told late Monday he would be allowed into his native America. Now a British citizen, Browder said he had been placed on an international police watch list and blocked from a flight to the U.S., and he accused the Russian government of being behind it.

Successfully lobbied Canada

He celebrated his apparent reprieve late Monday. Browder tweeted: "GREAT NEWS! My ... [US visa waiver] was restored. I successfully checked into a U.S. flight. Now we need to fix bogus Interpol arrest warrant."

The drama started after he successfully lobbied Canada to become the latest country to adopt a so-called Magnitsky law that targets corrupt officials of the Putin regime. He accused the Russian government of retaliating by placing his name on an Interpol watch list.

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Nataliya Magnitskaya (left), the mother of Sergei Magnitsky, grieves over her son's body during his funeral at a cemetery in Moscow on Nov. 20, 2009. Magnitsky died of heart failure in prison, state prosecutors said. (Mikhail Voskresensky/Reuters)

"United [Airlines] wouldn't let me board a flight," Browder tweeted earlier Monday. "Unless Interpol lifts this notice, I will be arrested at any international border I cross on Putin's orders."

These developments were simply the latest twist in a longer, darker saga involving him and the Russian leader.

Turned on Putin

Browder was a major hedge-fund manager in Russia in the early 2000s who complained about oligarchs stealing from companies he'd invested in. Initially, Putin was his ally — Browder's complaints about corruption would be followed by arrests.

The arrests stopped. Browder accuses Putin of becoming personally involved in corruption schemes, building a massive multibillion-dollar fortune, and turning on him. A crusading young lawyer working to investigate corruption for him, Sergei Magnitsky, was suddenly arrested by some of the very people he'd investigated. He was beaten repeatedly in prison and eventually died.

Browder has since made it his life's mission to avenge his friend's death. He has succeeded in getting several countries to pass laws in Magnitsky's honour. Canada adopted its law a few days ago.

The Canadian law allows authorities to block financial services for foreigners complicit in extrajudicial killings, torture, or other human-rights violations in efforts to protect a government that abuses basic rights.

McCain calls Browder a champion

Putin angrily responded to a similar law in the U.S. by stopping adoptions of Russian orphans. The issue has been brought up at the highest levels. The White House says President Donald Trump's son, son-in-law, and campaign manager discussed it with a Russian lawyer last year at Trump Tower.

Browder says he's often been the victim of Russian attempts to get him placed on Interpol wanted lists and says that this time he's been accused of murder — of killing his friend Magnitsky.

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Sen. John McCain issued a statement on Monday in support of Bill Browder and his work fighting corruption in Russia. (Aaron P. Bernstein/Reuters)

American lawmakers from both parties are demanding answers. Senator John McCain called Browder a champion of anti-corruption fighters in Russia and called for an immediate U.S. review of his status earlier Monday.

"We relied on his expertise and support as we led the effort to pass the Magnitsky Act," McCain said in a statement.

Unclear whether denial was accidental

"Mr. Browder's work has helped to remove corrupt actors from our financial system and enhance accountability measures with respect to the U.S. relationship with the Russian Federation — it would be unfortunate if the U.S. decided to bar him based on a decision by those same Russian officials who have been targeted by this important legislation."

It's unclear whether his earlier travel denial was accidental.

Browder said he had an electronic document allowing travel from visa-free countries, via the Electronic System for Travel Authorization. In a statement, the U.S. State Department said, "We have no record of this individual ever applying for a U.S. visa."

The statement noted that many United Kingdom citizens are eligible to travel via the visa waiver, but if they lose eligibility to use the visa-waiver program they must apply for a visa at a U.S. consulate or embassy.

With files from CBC News