Russia on Monday accused Ukraine of organising the murder of a prominent war blogger in a St Petersburg cafe and arrested a young Russian woman who was shown in a police video admitting planting the bomb that killed him and injured over 30 others.

Ukraine, which did not take responsibility for Sunday’s attack, blamed “domestic terrorism” for the murder of Maxim Fomin, a Russian military blogger and cheerleader for Russia’s invasion of Ukraine who called himself Vladlen Tatarsky.

Darya Trepova, the 26-year-old Russian woman arrested over his murder, confessed in a video released by the interior ministry that she had planted the bomb that killed him.

But unconfirmed Russian media reports said she had told investigators that she had been set up and had not known she was carrying a bomb.

Tatarsky’s murder appeared to be the second assassination on Russian soil of a figure closely connected to the conflict in Ukraine, after the car bomb killing of Darya Dugina, daughter of a nationalist ideologue, outside Moscow last summer. Russia also accused Ukraine at the time. Kyiv denied involvement.

With over 500,000 followers on the Telegram messaging service that is popular in Russia, Tatarsky - who had himself fought in Ukraine in the past - mixed ultra-nationalist messaging with criticism of the way Moscow is prosecuting what it calls its “special military operation” in Ukraine.

Mourners braved a snowstorm to lay flowers outside the St Petersburg cafe where he was killed, with many saying they were upset and angry. Some Russian commentators said the bombing was the latest sign that violence related to the war in Ukraine is increasingly spilling onto Russian territory.

Russia’s National Anti-terrorism Committee (NAC) said Ukraine’s intelligence services had organised the bombing with help from supporters of jailed Kremlin critic Alexei Navalny.

That appeared to be a reference to the fact that the Russian woman arrested on Monday - Trepova - once registered to take part in an anti-Kremlin tactical voting scheme promoted by Navalny’s movement.

Navalny’s allies, who have fled abroad since their movement was branded extremist by the Kremlin, rejected the accusation, saying it was more likely that Russia’s own intelligence services were behind the killing.

In the interior ministry video, Trepova confessed to giving Tatarsky a small statue which was packed with the explosives that killed him, but declined to say immediately who had given her the statue. “Can I tell you later?” she is heard saying.

Interior ministry spokeswoman Irina Volk said that Trepova had been arrested in a rented flat in St Petersburg as part of an operation by the police and the FSB security service.

The Kremlin called Tatarsky’s murder a “terrorist act,” citing the statement from the NAC as evidence that Ukraine might have been behind the killing.

“The active phase of the investigation is now under way,” Kremlin spokesman Dmitry Peskov told reporters.

“We see quite vigorous steps to detain suspects. Let’s be patient and wait for the next announcements from our special services, which are working on this.”

Mykhailo Podolyak, a Ukrainian presidential adviser, alleged on Sunday night that the killing was part of “an internal political fight” in Russia which he likened to spiders eating each other in a jar.

He provided no evidence to back that assertion.

Unconfirmed Russian media reports said that Trepova had bought plane tickets to flee to Uzbekistan. Reuters could not immediately confirm that detail.

Trepova had appeared on an interior ministry wanted list earlier on Monday. Court records showed she was detained on February 24 of last year, the day Russia launched a full-scale invasion of Ukraine, for taking part in what the authorities deemed an illegal anti-war protest.

The FSB said last month that it had thwarted a Ukraine-backed car bomb attack on a prominent nationalist businessman who has been a cheerleader for Moscow’s war in Ukraine.

Margarita Simonyan, editor-in-chief of state media outlet RT, welcomed Trepova’s arrest on Monday, saying her detention had avoided what she called “a national disgrace”.

Simonyan, like other hawkish commentators, made it clear on Telegram that she wanted Russia to hit back hard against whoever had killed Tatarsky. “Well well. Are we going to forget and forgive this?” she asked sarcastically.