The slaying of opposition leader Boris Nemtsov has been described as "the most significant political murder in recent Russian history." But the case remains steeped in mystery. As investigators home in on five suspects, dozens of versions of what motivated the killers have circulated across Russian media and the country's notoriously conspiratorial blogosphere.

Here's a look at the suspects and the theories:

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THE SUSPECTS

Investigators have detained five men in connection to Nemtsov's killing, all of them from Chechnya or other parts of Russia's predominantly Muslim North Caucasus.

Prime suspects Zaur Dadaev and Anzor Gubashev were charged in connection to the shooting. A judge said Dadaev had confessed to investigators, though he did not admit guilt in court.

The three others, who included Gubashev's younger brother Shagid, Khamzad Bakhaev and Tamerlan Eskerkhanov, have been detained but not charged.

WHO ORDERED THE CRIME?

The Kremlin-controlled media suggested that the suspects had acted for religious reasons. Meanwhile, the opposition believes that the government is responsible for Nemtsov's death — and railed against investigators for failing to pursue the person who had ordered the killing.

"The attempt to convince the public that Nemtsov was an obvious target for Islamic radicals doesn't stand up to criticism," opposition leader Ilya Yashin told The Associated Press on Monday.

"This version is extremely convenient for Vladimir Putin, because it takes both him and his inner circle out of the line of fire."

On Tuesday, respected independent daily Novaya Gazeta reported that investigators had a suspected mastermind: a Chechnya native with family connections to the upper echelons of Russian politics they named only as "Ruslan." The report said investigators had informed the president's office about the suspect, and guesses as to his identity circled across the Russian Internet.

STATE TELEVISION'S TAKE

In the hours after Nemtsov's death, Russian state television — where a majority of people get their news — rushed to cover his killing. They suggested several possible motives, from a vengeful person in his personal life to a foreign plot to destabilize Russia — every possibility other than that Nemtsov had been killed for his political activity.

In televised remarks on March 4, Putin called the killing a "disgrace" and urged state employees to prevent political murders in the future. In recent days, however, the investigation into Nemtsov's death has dwindled to the very bottom of news bulletins, sometimes dropping off them completely.