There has long been suspicion that the explosions in Bulgaria, at least those from 2015, were acts of sabotage. Why prosecutors are choosing to relaunch their investigation now is unclear.

Unlike the Czech authorities, who revealed new details about the explosions there and expelled dozens of Russian diplomats in response, Ms. Mileva provided little new evidence and made no indication that a response was forthcoming.

A fire that broke out at an administrative building in Sofia, the capital, in May 2015 destroyed evidence related to those two blasts, Ms. Mileva said.

Bulgaria’s investigation of the explosions comes at a time of escalating confrontation between Russia and the West. For weeks, Russian troops were massing on the border with Ukraine, though in the last week they have somewhat pulled back. This month the United States announced that it would expel 10 Russian diplomats and impose sanctions as punishment for a huge breach of government computers that the White House blamed on Russia’s foreign intelligence service.

Bulgaria, despite being a European Union member, has long maintained friendly relations with Russia, which is a critical energy supplier. But recently, there has been evidence that Bulgarian officials have grown weary of playing host to Russian intelligence operations.

In January 2020, Bulgarian authorities announced criminal charges against three officers from Unit 29155 for poisoning Mr. Gebrev, his son and the senior Emco executive. The three fell ill in April 2015, less than two weeks after one of the blasts at a Bulgarian ammunition depot. An investigation determined that they were sickened with a substance similar to the Novichok nerve agent that British authorities say was used by officers from Unit 29155 on Mr. Skripal and his daughter in the United Kingdom.

Last month, after Bulgarian officials announced the arrest of six people they said were involved in an espionage ring run by the Russian security services, the country’s prime minister, Boiko Borisov, spoke to reporters, telling the Kremlin to knock it off.

“Stop spying in Bulgaria,” Mr. Borisov said.

Boryana Dzhambazova reported from Sofia, and Michael Schwirtz from New York.

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