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MADRID (AP) — Spanish authorities have arrested a retired man on suspicion he sent six letters containing explosive material to Spain’s prime minister and the U.S. and Ukrainian embassies in the country, police said in a statement.

The 74-year-old Spanish national was arrested in the northern city of Miranda de Ebro and was “very active” on social media, police said. They added that authorities “do not rule out the participation or influence of other people” in the case, and a search of the man’s home was still under way.

An employee at the Ukrainian embassy in Madrid was slightly injured while handling a letter bomb in November. A bomb squad also destroyed an explosive device that was dispatched by regular post to Spanish Prime Minister Pedro Sánchez in the same month.

Letters with similar characteristics were sent to Spain’s Defense Ministry, a European Union satellite center located at the Torrejón de Ardoz air base outside Madrid and an arms factory in northeastern Spain that makes grenades sent to Ukraine.

An envelope intercepted at the American Embassy’s security screening point in December was detonated by authorities after a wide area was cordoned off by Spanish police around the embassy in the center of Spain’s capital.