At least one of the North Caucasus suspects in Nemtsov's killing, Zaur Dadayev, is said to have confessed to involvement in the February 27 assassination of opposition politician Boris Nemtsov in central Moscow.

The remaining four have denied participation, a claim that strengthens concerns that Moscow is looking to the restive North Caucasus as a credible scapegoat for a crime that many suspect is tied directly to the Kremlin.

Following the killing, Moscow authorities were quick to suggest Nemtsov's killing might have been committed by Muslims angered by his defense of the French newspaper Charlie Hebdo for its publication of cartoons mocking the Prophet Muhammad.

But Nemtsov ally Ilya Yashin has dismissed Islamist motives as a "nonsensical theory" by investigators tasked with keeping Russian President Vladimir Putin "out of the firing line."

To be certain, North Caucasus militant groups have claimed responsibility for a vast number of crimes, including the Beslan and Nord-Ost hostage massacres, Moscow subway bombings in 2004 and 2010, and synchronized suicide attacks on two Russian passenger planes in 2004.

In other instances, however, Chechens and other North Caucasians have been used as either convenient scapegoats or low-level, convictable trigger men who are believed to take the fall for the person who ordered the crime. We look at three notable examples:

1. Moscow apartment bombings

In September 1999, nearly 300 civilians were killed in a series of nighttime explosions that destroyed apartment blocks in Moscow, Volgodonsk, and Buinaksk. Within days, then-Prime Minister Vladimir Putin initiated the second Chechen War, claiming North Caucasus separatists were behind the blasts.

Militant leader Ibn al-Khattab denied the charge, saying separatists were opposed to targeting innocent civilians. Several State Duma lawmakers also doubted the claim, calling for an open parliamentary inquiry into the bombings amid growing evidence the bombings had been organized by the Federal Security Service (FSB).

But by 2002, Kremlin investigators had officially charged North Caucasian militants with the bombing. Seven of the suspects, including Khattab, were eventually killed. Another six were convicted on terrorism-related charges. At least four Russians who continued to assert FSB involvement later died in mysterious circumstances, including journalist Anna Politkovskaya and FSB whistle-blower Aleksandr Litvinenko.

2. Paul Klebnikov

Shortly after being appointed the first editor of the Russian edition of Forbes magazine, Klebnikov was attacked by armed gunmen outside his office building in July 2004. Authorities said the killing was a response to Klebnikov's investigative work, which included books on Russian tycoon Boris Berezovsky and Chechen rebel leader Khozh-Akhmed Nukhayev.

Prosecutors later accused Nukhayev of masterminding the crime and arrested three Chechen men for carrying out the hit. All three men were eventually acquitted and authorities said they no longer believed Nukhayev had ordered the killing.

3. Anna Politkovskaya

A journalist and activist, Politkovskaya had spent years documenting human rights abuses in Chechnya , crimes for which she blamed both the Kremlin and Chechen leader Ramzan Kadyrov. Politkovskaya was shot dead in the entryway of her building in a professional-style hit on Putin's birthday, October 7, 2006.

A year later, police arrested 10 suspects, including members of a Chechen organized crime group, whose aim, according to Prosecutor-General Yury Chaika, was to "destabilize" Russia. Investigators later accused Rustam Makhmudov, a 30-year-old Chechen whose two brothers were among the other suspects, of carrying out the hit. Makhmudov, whom opposition journalists accused of enjoying the protection of the FSB, later escaped to Europe.

After a 2008 trial, three Chechen defendants were acquitted of murder. Their verdict was overturned and in 2014, five Chechens -- including the original three defendants, Rustam Makhmudov, and an uncle, Lom-Ali Gaitukayev -- were convicted. A former policeman, Russian Dmitry Pavlyuchenkov, was convicted separately as the organizer of the hit. The mastermind has never been named; Politkovskaya's family says the case remains "unsolved."