Three ETA militants pose in front of the group's symbol of a snake coiled around an axe, in an undated file photo. NEW: Antonio Troitino Arranz's arrest deals a blow to ETA, the Spanish Interior Ministry says Troitino was responsible for more than 20 murders in terror attacks, it says London (CNN) -- Two members of the Basque separatist movement ETA, including one who has been convicted of more than 20 murders, were arrested Friday in London over alleged terror offenses, the Spanish Interior Ministry said. Troitino has been part of the Basque separatist movement ETA since the 1980s and was involved in many terrorist attacks, the ministry said. One of the worst was the bombing of a police bus in Madrid that left 12 officers dead and injured 51 people, including civilians, it said. According to Spanish media reports, Troitino was let out of jail due to what is described as a judicial error, and fled before it could be corrected and he could be detained again. Ignacio Lerin, who has been on the run since 2007, when his brother was arrested as an ETA member, is wanted on suspicion of belonging to an armed group and possession of explosives, it said. The arrests have dealt a blow to ETA, the ministry said, as Troitino has direct links to the group's present leadership. In London, officers continue to search the house in Hounslow where the two were arrested before dawn, a Metropolitan Police statement said. The two suspected ETA members, whose arrests were made under extradition law, are being held at a central London police station. Spanish security forces have arrested 16 members of ETA since January, including three in the past week, the Interior Ministry said. Spain's conservative government has stepped up arrests since winning elections in November. The Popular Party of Prime Minister Mariano Rajoy has long maintained a tough line against ETA. CNN's Alexander Felton and Al Goodman contributed to this report.