From Islamist extremists to spurned lovers to Russian President Vladimir Putin, speculation continues to swirl about who killed opposition leader Boris Nemtsov last week, a stone's throw from the Kremlin, and why.

Igor Eidman, Nemtsov's cousin, pins the high-profile assassination squarely on Putin.

But he suspects it was a slur used by the 55-year-old politician to describe Putin in a recent interview -- rather than Nemtsov's long-standing criticism of his policies -- that may have prompted the president to order the hit.

"For people with a criminal mentality like Putin's, such words can mean a lot more than concrete actions," Eidman tells RFE/RL.

Eidman says that in an interview with a Ukrainian television station, Nemtsov used a word -- yobnuty (ебнутый) -- to describe Putin that roughly translates as "f***ed up."

"A criminal boss cannot tolerate verbal abuse, otherwise he loses his authority. When Boris pronounced this word, my first thought was that he could be killed," Eidman says.

Just hours after his cousin's assassination, Eidman published an angry open letter that pointed the finger at Putin and dismissed theories that Russian ultranationalists gunned down Nemtsov for vocally denouncing Moscow's support of separatist insurgents in eastern Ukraine.

"Don't you know all our 'extremists' are closely watched and entirely controlled by the FSB?" he wrote. "The hand of the killer was guided by the Russian secret services. And only Putin could have given the order for such an assassination."

Eidman also gives credence to suspicions that Nemtsov could have been killed to prevent the release of a damning report he was preparing on Russia's actions in Ukraine, titled Putin And The War.

"The publication of such a report could have seriously undermined Putin's secret war," he wrote in his letter.

Eidman, a sociologist, political commentator and occasional blogger for RFE/RL's Russian Service, has rarely spoken publicly about his cousin, although the two men were close.

Nemtsov had studied physics with Eidman's father, a well-known professor who himself had studied under world-famous physicist Vitaly Ginzburg, a Nobel laureate and one of the fathers of the Soviet hydrogen bomb. In the last decade of his life, Ginzburg was also a vocal opposition supporter.

"He was a very talented physicist, a star," says Eidman of Nemtsov. "At the Scientific Research Institute, where he worked and presented his papers, he was one of the youngest -- if not the youngest -- and most promising scientists."

He clearly remembers Nemtsov's first foray into public life, in the late 1980s, when he joined the lobbying effort against plans to build a nuclear plant close to his hometown of Nizhny Novgorod, then called Gorky.

With the 1986 Chornobyl nuclear disaster still fresh in people's mind, the protests drew strong popular backing and thrust the young Nemtsov onto the public scene.

His clout as an environmental and civic activist quickly grew.

"People started turning to him," Eidman recalls. "He was young, he was dynamic, interesting, smart, and he was an eloquent speaker."

After unsuccessfully running for a seat in the newly formed U.S.S.R. Congress of People's Deputies in 1989, Eidman says Nemtsov seriously considered abandoning politics and resuming his scientific career. He says Nemtsov's wife at the time, Raisa, was also fiercely opposed to his entering politics.

Strongly encouraged by his friends and supporters, including Eidman himself, Nemtsov eventually decided to run for a seat in the Russian legislature in 1990, this time with success.

After a meteoric rise to power and a stint as Nizhny Novgorod governor, Russian President Boris Yeltsin named him first deputy prime minister in 1997, aged just 37.

Eidman says rumors that Yeltsin was grooming him for the presidency are not exaggerated.

"He had high ratings at the time and, it seemed, real chances," he says. "In addition, Yeltsin gave him all sorts of photographs with messages such as 'I hand over the reins of power' that clearly hinted he saw him as a successor."

The advent of Vladimir Putin to the presidency gradually drove Nemtsov into the opposition and, Eidman believes, ultimately caused his death.

"Putin is a bandit," he rages in his letter.

The investigators' suggestion that Nemtsov was used as a "sacrificial lamb" in a plot to discredit the Kremlin also riles Eidman.

"What's awful is that this murder cannot discredit Vladimir Putin," he tells RFE/RL. "Putin is already so hopelessly discredited in the eyes of the world that one more crime will not change anything about his reputation, which he has destroyed with his own hands."