ST. LOUIS – An area of St. Louis is back open following a gas leak Monday.

Three buildings along Chouteau Avenue and Seventh Street were evacuated, including one school, as first responders worked to clear the gas and its smell from the area.

“Biggest challenge is finding where it’s coming from and where it’s moving to,” St. Louis Fire Department Chief Dennis Jenkerson said. “Gas—when it’s underground—migrates and follows the pipe chases. So, we have to quickly get into these buildings and see what their levels are.”

Purina was also one of the buildings evacuated, which usually holds around 3,000 employees and up to 80 animals, according to Bill Etling, senior VP of corporate affairs at Nestle Purina.

They’ll return to the office Tuesday.

According to Spire, the gas leak was contained and deemed safe again around 11:45 a.m. However, their crews will remain on scene to continue monitoring levels.