Four people were wounded on Tuesday in an armed attack at the headquarters of the Turkish Telecommunications Workers\' Union in the Turkish capital of Ankara, Turkish sources told Xinhua.Unknown suspects opened fire at the building by using automatic rifles, breaking windows of the building and damaging vehicles parked outside the premises.Ambulances and police officers rushed to the scene of the incident, and the injured were moved to hospital for receiving medical treatment. Meantime, an investigation into the attack was underway.According to local media reports, the union is scheduled to hold a meeting on March 30-31.