The man, 20, who works for a company that protects IT systems against breaches, was arrested yesterday, police cybersecurity chief Yavor Kolev said.

Finance minister Vladislav Goranov apologised to the country for the attack on servers at the tax agency (NRA). One researcher said it may have compromised nearly every adult’s personal records.

Kolev said investigations were still at an early stage and police were always looking into the possibility that others were involved.

Officers seized a laptop and other devices from the suspect’s office and home in Sofia and found encrypted data, Kolev added.

The attack on the tax agency happened at the end of June and compromised about three percent of its databases, officials said. A person claiming to be a Russian hacker emailed local media on Monday offering access to the stolen data.

Initially, the Bulgarian government believed that the Russians were punishing the country for buying American jets to replace their ageing MIGs