The explosion tore through the roof of West Fertilizer Co., charring much of the structure and sending massive flames into the air. A deafening boom echoed for miles.
It registered as a 2.1-magnitude earthquake on the U.S. Geological Survey website.
Brad Smith felt his house shake. It's 50 miles away from the plant.
"We didn't know exactly what it was," he said. "The forecast said a line of thunderstorms was going to come through. My wife and I looked up and wondered, 'Did it get here six hours early?' "
Local authorities are working with federal officials, including the Bureau of Alcohol, Tobacco, Firearms and Explosives, to determine the cause of the deadly explosion.
Though there are no indications of criminal activity, Waco Police Sgt. William Patrick Swanton said, it has not been ruled out yet.
It's unclear whether the plant had safety problems. But in 2006, the Environmental Protection Agency fined the company that ran the fertilizer plant $2,300 and told the owners to correct problems that included a failure to file a risk management program plan on time.
Seven years ago, the company had a complaint against it for a lingering smell of ammonia, the Texas Commission on Environmental Quality website shows.
West is about 75 miles south of Dallas and about 20 miles north of Waco.
The blast came as the nation remains on edge following the Boston Marathon bombing on Monday that killed three and left about 180 injured.
It also coincided almost exactly with the 20th anniversary of a fire in Waco that ended a federal agents' siege against members of the Branch Davidian sect. More than 80 sect members and some federal agents died.
That anniversary is Friday.