A fireworks factory in China where an explosion killed 20 people and injured 150 more was operating illegally, according to reports.Shockwaves from the blast were felt over three miles away, shattering building windows. The explosion happened on Monday near the city of Yichun in Heilongjiang province. It is unclear what caused the explosion but it is believed the blast occurred while workers were moving stock inside the factory. The factory had its licence confiscated in June after safety concerns were raised according to the official Xinhua news agency.An eyewitness from a nearby building said: "We assumed it was an earthquake because the office building trembled and the lamps and clocks fell. "A huge black mushroom cloud was visible. It rose and grew from where the fireworks factory is located." More than 2,000 residents were evacuated from the area as over 550 firefighters tried to extinguish the blaze.