The Russian army has joined the rescue operation. Army

units are searching for meteorite debris in several places in Chelyabinsk region and in the neighbouring Tyumen region. The Emergency Ministry reported that 20,000 rescue workers are operating in the region. Three aircraft were deployed to survey the area and locate other possible impact locations. Radiation, chemical and biological protection units have been put on high alert in the area. As the explosion has occurred several kilometres above ground, a large ground area has to be thoroughly checked for radiation and other threats. At least one piece of the fallen object caused damage on the ground in Chelyabinsk. According to local reports, it crashed into a wall near a zinc factory, disrupting the city's Internet and mobile service. Witnesses said the explosion was so loud that it seemed like an earthquake and thunder had struck at the same time, and that there were huge trails of smoke across the sky. Others reported seeing burning objects fall to earth. Chelyabinsk resident Sergei Serskov told BBC News the city had felt like a "war zone" for 20 to 30 minutes. "I was in the office when suddenly I saw a really bright flash in the window in front of me," he said. "Then I smelt fumes. I looked out the window and saw a huge line of smoke, like you get from a plane but many times bigger."