MOSCOW Islamist militants have targetted Russia many times before but Monday's bombing of a metro train in St Petersburg was, for President Vladimir Putin, personal: it happened in his native city on a day he was making a visit back home.
The attack, which killed 14 people and wounded 50, is also a test for one of Putin's most contentious policies - his decision to launch a military intervention in Syria on the side of President Bashar al-Assad.
Kremlin-watchers say the risk is that Russian voters could decide, after seeing the destruction in St Petersburg, that the Syrian operation is making them more vulnerable to such attacks - not safer as Putin promised.
That would be problematic for Putin, who faced a new wave of anti-corruption protests last month, before a presidential election next year when he is expected to seek a fourth term.
The main suspect in the blast, Akbarzhon Jalilov, is a Russian citizen from mainly Muslim Kyrgyzstan. No group has said it was behind the attack but in the past Islamic State has threatened to avenge Syria. It already said it was responsible for bringing down a planeload of Russian tourists over Egypt's Sinai peninsula in 2015, killing all 224 people on board.
If the St Petersburg bombing turns out to be Islamic State's response to Russia's operation in support of Assad, "that would signify the complete failure of Putin's gambit in Syria", said Alfred Kokh, a former deputy prime minister under the late president Boris Yeltsin.
"If you add to the mix the air disaster over Sinai ... then the picture is looking very grim for the author of Russia's participation in the Syria conflict," said Kokh.
However, it was not clear that ordinary Russian voters would view it as a Putin failure. Opinion polls give him a high level of support, and there was no discernable damage to his standing after the plane bombing over Egypt.
"Now we feel a blow, trauma, but actually with us trauma passes quite quickly," Gleb Pavlovsky, a former Kremlin adviser, told Russia's Ekho Moskvy radio station.
"In a week's time it will be clear which way people are inclining - towards what in the past was a customary demand for more repression, or towards fatigue over this perpetual situation where we are being sold security under various guises, but we still don't have security."
Early indications are that the Kremlin will respond by arguing that the St Petersburg attack underlines the importance of the operation in Syria, which Russia says is primarily aimed at crushing Islamist militants.
Another response may be to tighten security inside Russia and launch a crackdown on all manifestations of hardline Islam. That would make sense from the practical point of view of stopping further attacks.
But the bombing could produce a side benefit for the Kremlin. Russian officials were taken unawares last month when thousands of people turned out in cities across Russia to protest against alleged official corruption.
The organisers of the protests said they would intensify their activity as the 2018 election approaches.
"Will the security threat be mobilised as a pretext to ban demonstrations (in the interests of public safety, of course), or more broadly as a reason to say 'now is not the time for division'?" Mark Galeotti, an expert on Russian security, wrote in business publication bne IntelliNews.
(Editing by David Stamp)