Which of the following is irrelevant to the cause of the explosions?

All three bombs exploded within half an hour of each other outside a police station south of the city centre. A national newspaper received an anonymous phone call warning about the attack just minutes before the first device exploded in the early hours of the morning. A fourth device was found by the bomb squad and destroyed in a controlled explosion. The whole area has been cordoned off and the anti-terrorism unit of the police has begun an investigation. So far, the police have not said what kind of bombs were used in the attack and no one has admitted responsibility. But suspicion is bound to fall on radical left-wing organizations which have carried out simdilar attacks in the past.