More mercury detected inside Walmart, 4th business under evaluation

"We now know from law enforcement that the timeline did not begin on Saturday, but the time appears to have begun Friday the 13th," said Dr. David Perse.

Initially, authorities thought the Mercury was predominantly in the Walmart parking lot, the Sonic restaurant, and the Shell Gas Station on Westview and West Sam Houston Parkway.

"We found there's mercury inside the Walmart and in particular, there's a McDonald's inside the Walmart that had higher levels," Perse said. "Across the beltway at a Bucky's, we discovered there's mercury there as well."

Police say the person behind the mercury is 19-year-old Christopher Lee Melder and they say he stole it from a lab.

He was arrested by the FBI and is charged with burglary unlawful exposure to hazardous material and an outstanding drug possession warrant.

"We still believe the total amount of mercury is less than a pint," Perse said. "From a health standpoint, I'm not terribly concerned."