The gangland-style killing of Boris Nemtsov on Saturday marked the worrying return of political assassination to Russia's internal power games. While Russia has remained one of the most dangerous places to be a government opponent or outspoken journalist, murders of opposition members have actually tapered off in recent years.

Nemtsov was shot four times as he walked on a bridge across the Moskva river, only metres from the Kremlin walls, and just one day before he was due to lead a protest rally against Vladimir Putin. 

For Kremlin critics, the most likely culprits will be those close to Putin's inner circle, most of whom have backgrounds in the FSB. There is a fairly lengthy set of precedents for such an assumption. In 1998 Galina Staravoitova, the human rights campaigner and democratic politician, was gunned down outside her apartment in St Petersburg. The assassination's organisers were officially found in 2013, yet questions remain about the alleged role of organised crime gangs, rival politicians and the Russian security services in ordering the hit.

Sergey Yushenkov was shot dead outside his apartment in 2003. He had been investigating charges that the FSB was behind the pair of apartment bombings in Moscow during 1999 that were subsequently blamed on the Chechens.

Paul Klebnikov, the editor of Russia's Forbes magazine, was shot and killed in 2004 while working on a story about Kremlin corruption and embezzlement during the reconstruction of Chechnya.

In 2006, the anti-Putin journalist Anna Politkovskaya was shot dead at her home. And a month later, the Russian defector Alexander Litivinenko was sensationally poisoned with polonium in London. His death came after he had written a book alleging that the Kremlin was behind the bomb plots of 1999, which were subsequently used as justifications for war in Chechnya.

According to this prominent view, members of the siloviki – politicians and officials originally from the security services – have methodically been doing away with anyone who might challenge their privileged positions. This includes meddling in the affairs of other countries. A case in point was the poisoning of the anti-Russian Ukrainian presidential candidate Viktor Yushchenko with dioxin in 2004, which left him badly disfigured. And before his death Nemtsov was compiling a report which his supporters claim showed direct proof of Russian involvement in the conflict in Ukraine.

A second possibility accepts that the siloviki are responsible, but with the added caveat that they acted with Putin's blessing. In other words, Putin is directly complicit in Nemtsov's killing, and potentially in past murders too.

Why would Putin want to kill Nemtsov? For one thing, it would be a pre-emptive strike on any domestic unrest, sending a message to his opponents after he had just reluctantly reined in pro-Russian rebels at the Minsk 2 peace talks. Hence Putin's calculations would be driven by the need to show strength, to terrorise dissenters, and avoid the perception of weakness among Russia's sizeable far right.

But one wonders whether this can be true. After all, it runs counter to Putin's domestic interests. What could be gained from eliminating a marginalised critic, without even a seat in parliament, and stirring up internal unrest? Perhaps he could blame it on Ukrainian spies as a pretext for invasion? Yet he has accomplished more or less what he wants in the Ukraine. In truth, the optics of Nemtsov's killing, his body lying in the street with St Basil's as backdrop, are bad for Putin. The episode will automatically draw support away from the Kremlin, and a crackdown in the aftermath will only make that worse.

There is a third possibility: that fellow liberals, perhaps with links to Ukrainian nationalists, killed Nemtsov. In the lead-up to the last presidential election, Putin himself had issued a warning that anti-Kremlin figures could be murdered by their own kind. That would foment a crisis which could then be blamed on the government. If a Ukrainian fifth column was involved, the rationale would be that killing Nemtsov would spark a wider war with Ukraine, and thereby hasten Kiev's entry into NATO.

That sounds pretty fantastical, but we should not be too quick to rush to judgment. After all, there is now significant evidence that Ukrainian nationalists played a role in the violence unleashed at Kiev's Euromaidan, and that it was not just the security services to blame for violence. Hence it is possible (albeit unlikely) that Nemtsov was deliberately made a martyr.

On the face of it, though, it seems most likely that someone with government connections was ultimately responsible. The hit was well planned, the scene was a highly symbolic backdrop, and the target was an individual who had long been a thorn in the side of government officials. If so, it is unlikely Nemtsov's murder will be solved any time soon, since the culprit(s) would have been careful to distance themselves from the actual triggermen.

By the same token it is far too simplistic to blame 'the FSB'. Kremlin politics are extremely complex and virtually impossible to reliably track. There are four to seven competing factions, some of which overlap. All of them involve members who swap sides. Some are frequently tagged 'hardliners', like the groups controlled by Igor Sechin and Nikolai Patrushev. Others, like Sergei and Viktor Ivanov, are sometimes branded more moderate. But they are far from a united cabal, and Putin regularly moves individuals up and down to keep them on their toes. Any one of these groups, or their supporters, could have been involved, which makes getting to the bottom of the case even harder. Or it could have been none of them: some other highly plausible potential culprits include organised criminals and Russian ultranationalists. Each had a reason to kill Nemtsov, whether driven by the profit motive or by a hatred of liberal ideology.

Nemtsov knew he might be killed. He had spoken of the possibility as recently as a fortnight ago. Yet his death is more than just a sign that liberals are once again potential execution targets. It will be a vital test of opposition to Putin's rule inside Russia. Nemtsov's murder has already caused popular outrage, as evidenced by this morning's march in Moscow. If it sparks widespread civil disobedience, it will be a sign that Putin's seemingly immovable spot at the apex of Russian politics is vulnerable. But if the liberal intelligentsia accept it meekly and with only token protests, it will demonstrate the direct opposite: that there is still no realistic alternative to the government in the Kremlin.