Madrid, Dec 1 : The police in Spain are investigating four more incendiary devices and letter bombs, a day after one exploded at Ukraine's embassy in Madrid, according to a media report. The devices were sent to the Prime Minister, the Defence Ministry, an arms company that makes rocket launchers donated to Kiev, and a military airbase near the Spanish capital, as well as the one found at the embassy, The Guardian reported. The first letter bomb exploded when it was opened by an embassy employee on Wednesday, causing minor injuries to the worker's hands and leading Ukraine to warn its diplomats to bolster their security precautions. The second, discovered hours later at the Instalaza weapons company in Zaragoza in the Aragon region that manufactures C90 rocket launchers, was deactivated by bomb squad officers, The Guardian reported. In the early hours of Thursday, the police were called to the Torrejon de Ardoz airbase after security systems there detected a suspicious package. On Thursday morning, it emerged that an incendiary device, addressed to the prime minister, Pedro Sanchez, had been intercepted on November 24. According to the Interior Ministry, another device was sent to the Defence Ministry. It appeared to have been sent from somewhere in Spain, The Guardian reported. A spokesperson for the Interior Ministry said X-rays had shown the envelope sent to the Torrejon de Ardoz base contained some kind of mechanism. "Officers from both the Policia Nacional and the Guardia Civil went to the base to seal off the area and police investigators are analysing the envelope, which was addressed to the satellite centre," he said. Preliminary investigations suggest a link between the first two letters as both envelopes supposedly came from Ukraine and bore the same sender email address. /IANS