Tearful parents in shock after school shooting NEW: Residents describe the evacuation from the school after the deadly shooting NEW: A woman inside the school during the shooting describes "pop, pop, pop" NEW: Child: Teachers and police order children to hide in a corner "It doesn't seem possible," one parent says Newtown, Connecticut (CNN) -- The sound of gunfire, confusion, a lockdown and then an evacuation. Witnesses, students and parents told frightening stories Friday about a school shooting that a law enforcement source said left close to 30 people dead, most of them children. Many details of the attack remained unclear, but the sight of dozens of emergency vehicles and police spread across the wooded school campus made it clear Sandy Hook Elementary School has become the nation's latest infamous crime scene. Authorities said the school had been secured and that danger had passed. Parents, who had rushed to the school after hearing the news, reunited with their children, clutching them and then hurrying away, while police checked nearby houses and buildings. The FBI and the Bureau of Alcohol, Tobacco, Firearms and Explosives are on the scene. An American flag in front of the Sandy Hook firehouse, where children were evacuated, flew at half staff. Outside the school, Lynn Wasik wrapped her arms around her 8-year-old daughter, Alexis, cloaked against the cold in an oversized jacket. The girl described her ordeal after police and teachers barged into her third-grade classroom and ordered her and her classmates to hide in a corner. "Everybody was crying," Alexis said. And I just heard the police officers yelling. Her mother said she first learned about the emergency through an automated phone call message. She said the message wasn't clear about the school where the incident had occurred. In a panic, she raced to Sandy Hook, eventually finding Alexis unharmed. "My heart is in a million pieces for those families," said Lynn Wasik. Who could do something like this? It's just sickening. Like Wasik, other parents wrapped their arms around their children as they hurried away from the scene. The FBI presence became much more evident in the afternoon. Several federal officers in tactical gear were coordinating with state and local law enforcement. Teary-eyed parents continued to emerge from the firehouse. Some were talking on cell phones, using words like "chaotic" and "devastating." Others were openly weeping into their phones as they walked up a wooded roadway leading away from the school. Why?" one woman cried as she walked away. Earlier, a woman who lives near the firehouse described seeing "a bunch of children with a bunch of adults" apparently evacuating toward a nearby park. "I seen five children running up through our back field," the woman told CNN. And by God, those poor little guys were running. Breaking news photos from outside the school showed adults leading children away from the school. The youngsters wearing, no coats and dressed in brightly colored clothing, walked one behind another, with their hands resting on the shoulders of the ones in front of them. After an initial report that "close to 20 people" were killed in the school, a federal law enforcement source in Washington told CNN that the number was closer to 30, and most of those killed were children. A parent who was inside the school at the time of the shooting described hearing a "pop, pop, pop," sound around 9:30 a.m. Also in the room were the school's principal, vice principal and the psychologist. All three left the room and went into the hall to see what was happening. The parent ducked under the table and called 911. "I cowered," she told CNN's Meredith Artley. The shooter "must have shot a hundred rounds." Later the parent said she saw two adults lying dead in the hallway, in a pool of blood. "I can't believe -- in a small town like this -- we've never had anything like this happen," a father of a Sandy Hook student told local CNN affiliate WTNH. "It doesn't seem possible," said another parent. I guess I'm still in shock. . CNN's David Ariosto and Susan Candiotti reported from Newtown, Connecticut, and Thom Patterson and Meredith Artley reported from Atlanta.