Speculation flares after an influential Russian military blogger is killed by a bomb in St. Petersburg.

An influential Russian military blogger who called for an escalation of the war in Ukraine was killed when a bomb exploded in a cafe in St. Petersburg, Russia, on Sunday, in what appeared to be one of the most high-profile attacks on a supporter of Moscow’s invasion.

The blogger, Maksim Fomin, who was more popularly known as Vladlen Tatarsky, was giving a public talk in the center of Russia’s second-largest city when the explosion ripped through Street Food Bar #1 Cafe, the Russian Interior Ministry and investigative authorities said.

Videos posted on social media showed Mr. Tatarsky receiving a small statue in his likeness onstage shortly before the explosion. An independent local news outlet, Fontanka, cited a witness as saying the blogger had received the statue as a gift from a woman who introduced herself as a sculptor called Nastya.

Another witness said Mr. Tatarsky had asked the woman to bring the statue to him, after she said she had been told she could not take it inside because of bombing fears, according to the Russian tabloid Komsomolskaya Pravda. About 100 people had gathered at the cafe to listen to him speak about his experience as a military blogger, Fontanka said.

At least 25 other people were injured in the explosion, with 19 of them hospitalized, according to the city’s governor, Aleksandr Beglov.

Mr. Tatarsky represented a radical wing of pro-invasion bloggers and activists who backed Moscow’s war but also criticized what they saw as the flaws in the Russian Army. In recent months, he appeared to have grow increasingly pessimistic of Russia’s war prospects, saying that only a major overhaul could secure a victory.

He was critical of Russian military commanders and highlighted the problems of the Russian Army. In a recent video, he suggested that nothing would change if “you replace the defense minister or chief of the general staff.”

“We need to change the system,” he said.

His death was the most high-profile attack on a prominent war supporter inside Russia since August, when a car bomb killed Daria Dugina, the daughter of an ultranationalist Russian supporter of President Vladimir V. Putin’s.

United States intelligence officials later said they believed the attack had been authorized by parts of the Ukrainian government, which denied any involvement.

Mykhailo Podolyak, an adviser to President Volodymyr Zelensky of Ukraine, suggested that the attack was a sign of internal fractures in Russian society, in line with Kyiv’s usual description of acts of sabotage in Russia since the war began.

Mr. Podolyak wrote on Twitter: “Spiders are eating each other in a jar. Question of when domestic terrorism would become an instrument of internal political fight was a matter of time.”

Russian officials pointed the finger at the government in Kyiv, without directly accusing Ukraine of killing the blogger. Russia’s Foreign Ministry said in a statement that Mr. Tatarsky’s work had “caused hatred” of the Ukrainian government.

Sergei Neverov, deputy speaker of the Russian Parliament, said, “It is clear to everyone who is behind it.”