Seven contract workers were injured when a storage tank exploded Sunday morning at the Magellan Midstream Partners LP oil refinery in Corpus Christi, TX, local news organizations reported.

The contractors were cleaning a storage tank at the site when the vessel caught on fire and exploded, a company representative told the Associated Press said. Officials briefly issued a shelter-in-place order for the area near the plant, but the incident did not contaminate the air near the facility or cause damage to nearby property.

Workers involved in the incident are employed by industrial cleaning firm Shelton Services Inc.

“We are saddened by this event. We are praying for an supporting our injured colleagues and their families. All of us at Shelton Services hold safety as our highest priority,” the company said in a statement to television news station KHOU. Shelton Services said it is conducting its own investigation into the cause of the incident.

Four of the seven workers involved in the blast were critically injured. Two of the contractors were later released from an area hospital, according to KIII TV.

“The State of Texas is working closely with Magellan and Corpus Christi officials to aid in the emergency response effort and to provide immediate help to those injured in the explosion,” Texas Gov. Greg Abbott said in a statement printed by KSAT. “The Texas Commission on Environmental Quality is monitoring air quality in the area, and the Texas Division of Emergency Management is on the ground to provide support.”